test(config): add unit test coverage for the remaining config objects (apache#6094)

### What changes were proposed in this PR?

Add unit test coverage for the remaining `common/config` objects,
selected from the Codecov report (all 0%). No production-code changes.

| File | Codecov before | What the tests pin |
| --- | --- | --- |
| `UdfConfig.scala` | 0% | python/R path + log handler defaults from
udf.conf |
| `DefaultsConfig.scala` | 0% | the `reinit` flag and the flattened
`allDefaults` short-key/value map from default.conf |
| `ComputingUnitConfig.scala` | 0% | local/sharing enabled flags |
| `LLMConfig.scala` | 0% | LiteLLM base URL + master key |
| `PekkoConfig.scala` | 0% | actor/serialization, remote/artery, and
cluster/failure-detector settings from cluster.conf (no ActorSystem
started) |
| `PythonUtils.scala` | 0% | `getPythonExecutable` blank→`python3`
fallback and trimmed-path branch |

Reading each value forces resolution from the backing `.conf`;
env/system-property-overridable values are guarded on the override being
unset (mirroring `StorageConfigSpec`).

### Any related issues, documentation, discussions?

Follow-up to the review feedback on apache#6043: prioritize tests that fill
uncovered code paths.

### How was this PR tested?

- `sbt "Config/testOnly *UdfConfigSpec *DefaultsConfigSpec
*ComputingUnitConfigSpec *LLMConfigSpec *PekkoConfigSpec
*PythonUtilsSpec"` — 13 tests, all green
- `sbt "Config/Test/scalafmtCheck"` and `sbt "Config/scalafixAll
--check"` — clean

### Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Opus 4.8 [1M context])

Author: aglinxinyuan

common/config/src/test/scala/org/apache/texera/common/config/ComputingUnitConfigSpec.scala

@@ -0,0 +1,43 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.common.config
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+/**
+  * Spec for [[ComputingUnitConfig]]. Reading each value forces resolution from computing-unit.conf,
+  * so a renamed or mistyped key surfaces here as a ConfigException. Both flags carry a `${?ENV}`
+  * override, so exact-value assertions are guarded.
+  */
+class ComputingUnitConfigSpec extends AnyFlatSpec with Matchers {
+
+  private def ifUnset(name: String)(assertion: => Any): Unit =
+    if (!sys.env.contains(name) && !sys.props.contains(name)) assertion
+
+  "ComputingUnitConfig" should "resolve the local/sharing flags from computing-unit.conf" in {
+    ifUnset("COMPUTING_UNIT_LOCAL_ENABLED")(
+      ComputingUnitConfig.localComputingUnitEnabled shouldBe true
+    )
+    ifUnset("COMPUTING_UNIT_SHARING_ENABLED")(
+      ComputingUnitConfig.sharingComputingUnitEnabled shouldBe false
+    )
+  }
+}

common/config/src/test/scala/org/apache/texera/common/config/DefaultsConfigSpec.scala

@@ -0,0 +1,52 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.common.config
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+/**
+  * Spec for [[DefaultsConfig]]. Reading each value forces resolution from default.conf, so a
+  * renamed or mistyped key surfaces here as a ConfigException. The `reinit` flag carries a
+  * `${?ENV}` override, so its exact-value assertion is guarded.
+  */
+class DefaultsConfigSpec extends AnyFlatSpec with Matchers {
+
+  private def ifUnset(name: String)(assertion: => Any): Unit =
+    if (!sys.env.contains(name) && !sys.props.contains(name)) assertion
+
+  "DefaultsConfig.reinit" should "default the reset-to-defaults flag to false" in {
+    ifUnset("CONFIG_SERVICE_ALWAYS_RESET_CONFIGURATIONS_TO_DEFAULT_VALUES")(
+      DefaultsConfig.reinit shouldBe false
+    )
+  }
+
+  "DefaultsConfig.allDefaults" should "flatten default.conf into short-key/value entries" in {
+    val defaults = DefaultsConfig.allDefaults
+    defaults should not be empty
+    // scalar leaves are flattened to their last path segment
+    ifUnset("DATASET_SINGLE_FILE_UPLOAD_MAX_SIZE_MIB")(
+      defaults.get("single_file_upload_max_size_mib") shouldBe Some("20")
+    )
+    ifUnset("GUI_TABS_HUB_ENABLED")(defaults.get("hub_enabled") shouldBe Some("true"))
+    // every value is rendered as a String
+    defaults.values.foreach(_ shouldBe a[String])
+  }
+}

common/config/src/test/scala/org/apache/texera/common/config/LLMConfigSpec.scala

@@ -0,0 +1,39 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.common.config
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+/**
+  * Spec for [[LLMConfig]]. Reading each value forces resolution from llm.conf, so a renamed or
+  * mistyped key surfaces here as a ConfigException. Both values carry a `${?ENV}` override, so
+  * exact-value assertions are guarded.
+  */
+class LLMConfigSpec extends AnyFlatSpec with Matchers {
+
+  private def ifUnset(name: String)(assertion: => Any): Unit =
+    if (!sys.env.contains(name) && !sys.props.contains(name)) assertion
+
+  "LLMConfig" should "resolve the LiteLLM base URL and master key from llm.conf" in {
+    ifUnset("LITELLM_BASE_URL")(LLMConfig.baseUrl shouldBe "http://0.0.0.0:4000")
+    ifUnset("LITELLM_MASTER_KEY")(LLMConfig.masterKey shouldBe "")
+  }
+}

common/config/src/test/scala/org/apache/texera/common/config/PekkoConfigSpec.scala

@@ -0,0 +1,79 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.common.config
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+/**
+  * Spec for [[PekkoConfig]]. Reading keys off the returned config forces resolution of cluster.conf
+  * (merged with the Typesafe default application config), so a renamed or mistyped key surfaces here
+  * as a ConfigException. The log level carries a `${?ENV}` override, so its assertion is guarded.
+  */
+class PekkoConfigSpec extends AnyFlatSpec with Matchers {
+
+  "PekkoConfig.pekkoConfig" should "load the actor/serialization settings from cluster.conf" in {
+    val config = PekkoConfig.pekkoConfig
+    config.getString("pekko.actor.provider") shouldBe "cluster"
+    config.getBoolean("pekko.actor.allow-java-serialization") shouldBe false
+    config.getBoolean("pekko.actor.enable-additional-serialization-bindings") shouldBe true
+    config.getString(
+      "pekko.actor.serializers.kryo"
+    ) shouldBe "io.altoo.serialization.kryo.pekko.PekkoKryoSerializer"
+    config.getStringList("pekko.loggers").get(0) shouldBe "org.apache.pekko.event.slf4j.Slf4jLogger"
+    config.getString(
+      "pekko.logging-filter"
+    ) shouldBe "org.apache.pekko.event.slf4j.Slf4jLoggingFilter"
+  }
+
+  it should "expose the remote/artery transport settings" in {
+    val config = PekkoConfig.pekkoConfig
+    config.getString("pekko.remote.artery.transport") shouldBe "tcp"
+    config.getString("pekko.remote.artery.canonical.hostname") shouldBe "0.0.0.0"
+    config.getInt("pekko.remote.artery.canonical.port") shouldBe 0
+    config.getBytes("pekko.remote.artery.advanced.maximum-frame-size") shouldBe 31457280L
+    config.getBytes("pekko.remote.artery.advanced.maximum-large-frame-size") shouldBe 125829120L
+  }
+
+  it should "expose the cluster and failure-detector settings" in {
+    val config = PekkoConfig.pekkoConfig
+    config.getStringList("pekko.cluster.seed-nodes").size shouldBe 0
+    config.getString(
+      "pekko.cluster.downing-provider-class"
+    ) shouldBe "org.apache.pekko.cluster.sbr.SplitBrainResolverProvider"
+    config.getString("pekko.cluster.gossip-interval") shouldBe "10s"
+    config.getString("pekko.cluster.failure-detector.acceptable-heartbeat-pause") shouldBe "50s"
+    config.getString(
+      "pekko-kryo-serialization.kryo-initializer"
+    ) shouldBe "org.apache.texera.amber.engine.common.AmberKryoInitializer"
+  }
+
+  it should "resolve the log levels" in {
+    val config = PekkoConfig.pekkoConfig
+    // ${?TEXERA_SERVICE_LOG_LEVEL} can be satisfied by an OS env var or a JVM system property
+    if (
+      !sys.env
+        .contains("TEXERA_SERVICE_LOG_LEVEL") && !sys.props.contains("TEXERA_SERVICE_LOG_LEVEL")
+    ) {
+      config.getString("pekko.loglevel") shouldBe "INFO"
+    }
+    config.getString("pekko.stdout-loglevel") shouldBe "INFO"
+  }
+}

common/config/src/test/scala/org/apache/texera/common/config/PythonUtilsSpec.scala

@@ -0,0 +1,52 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.common.config
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+/**
+  * Spec for [[PythonUtils]]. `getPythonExecutable` falls back to "python3" when the configured
+  * python path is blank, else returns the trimmed path. The blank-path default depends on
+  * UDF_PYTHON_PATH, so the exact-value assertion is guarded on that override being unset.
+  */
+class PythonUtilsSpec extends AnyFlatSpec with Matchers {
+
+  "PythonUtils.getPythonExecutable" should "fall back to python3 when no python path is configured" in {
+    // ${?UDF_PYTHON_PATH} can be satisfied by an OS env var or a JVM system property
+    val overrideValue =
+      sys.env.get("UDF_PYTHON_PATH").orElse(sys.props.get("UDF_PYTHON_PATH"))
+    if (overrideValue.forall(_.trim.isEmpty)) {
+      PythonUtils.getPythonExecutable shouldBe "python3"
+    }
+  }
+
+  it should "never return a blank or untrimmed executable" in {
+    val executable = PythonUtils.getPythonExecutable
+    executable should not be empty
+    executable shouldBe executable.trim
+  }
+
+  it should "match its own fallback logic against the backing UdfConfig value" in {
+    val expected =
+      if (UdfConfig.pythonPath.trim.isEmpty) "python3" else UdfConfig.pythonPath.trim
+    PythonUtils.getPythonExecutable shouldBe expected
+  }
+}

common/config/src/test/scala/org/apache/texera/common/config/UdfConfigSpec.scala

@@ -0,0 +1,53 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.common.config
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+/**
+  * Spec for [[UdfConfig]]. Reading each value forces resolution from udf.conf, so a renamed or
+  * mistyped key surfaces here as a ConfigException. Every value carries a `${?ENV}` override, so
+  * exact-value assertions are guarded on the override being absent from env and system properties.
+  */
+class UdfConfigSpec extends AnyFlatSpec with Matchers {
+
+  private def ifUnset(name: String)(assertion: => Any): Unit =
+    if (!sys.env.contains(name) && !sys.props.contains(name)) assertion
+
+  "UdfConfig" should "resolve the python and R defaults from udf.conf" in {
+    ifUnset("UDF_PYTHON_PATH")(UdfConfig.pythonPath shouldBe "")
+    ifUnset("UDF_R_PATH")(UdfConfig.rPath shouldBe "")
+    ifUnset("UDF_PYTHON_LOG_STREAMHANDLER_LEVEL")(
+      UdfConfig.pythonLogStreamHandlerLevel shouldBe "INFO"
+    )
+    ifUnset("UDF_PYTHON_LOG_FILEHANDLER_DIR")(UdfConfig.pythonLogFileHandlerDir shouldBe "/tmp/")
+    ifUnset("UDF_PYTHON_LOG_FILEHANDLER_LEVEL")(UdfConfig.pythonLogFileHandlerLevel shouldBe "INFO")
+  }
+
+  it should "resolve the non-empty log format strings" in {
+    ifUnset("UDF_PYTHON_LOG_STREAMHANDLER_FORMAT")(
+      UdfConfig.pythonLogStreamHandlerFormat should not be empty
+    )
+    ifUnset("UDF_PYTHON_LOG_FILEHANDLER_FORMAT")(
+      UdfConfig.pythonLogFileHandlerFormat should not be empty
+    )
+  }
+}