test(imports): isolate remote-importer cache to a temp dir

TestResolveImportPaths_MixedPaths flaked on the Windows CI runner:

  open C:\...\cache\atmos\stack-imports\<hash>.yaml:
  The system cannot find the path specified.

The remote-import tests drive the package-global importer, which lazily
builds a real-user-cache-backed importer via getGlobalImporter() ->
globalImporterOnce.Do(). The tests tried to isolate it by assigning a
temp-cache importer after `globalImporterOnce = sync.Once{}`, but a freshly
reset Once still runs on the next getGlobalImporter() call and overwrites the
injection with a real-cache importer. So every remote-import test actually
wrote to (and read back from) the shared user cache dir — which races/flakes
on CI runners (Windows most visibly).

Add useTestGlobalImporter(t, cfg) which primes the Once (consumes it via
Do) so the injected temp-cache importer survives, and restores the globals on
cleanup. Route all remote-import tests through it.

Verified: with XDG_CACHE_HOME pointed at an empty dir, the package no longer
writes any stack-imports/*.yaml there — downloads stay in t.TempDir().

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: osterman

pkg/stack/imports/remote_test.go

@@ -40,6 +40,28 @@ func newTestRemoteImporter(t *testing.T, atmosConfig *schema.AtmosConfiguration)
 	return importer
 }
 
+// useTestGlobalImporter swaps the package-global remote importer for one backed by an
+// isolated temp cache dir, and restores it on cleanup. It must prime globalImporterOnce
+// (not just assign globalImporter): getGlobalImporter() calls globalImporterOnce.Do, so a
+// freshly-reset Once would otherwise run and overwrite the injection with a real-cache
+// importer that writes to the shared user cache dir — which flakes on CI runners
+// (notably Windows: "The system cannot find the path specified").
+func useTestGlobalImporter(t *testing.T, atmosConfig *schema.AtmosConfiguration) {
+	t.Helper()
+	importer := newTestRemoteImporter(t, atmosConfig)
+
+	globalImporterOnce = sync.Once{}
+	globalImporterErr = nil
+	// Consume the Once now so the lazy getGlobalImporter() keeps our injected importer.
+	globalImporterOnce.Do(func() { globalImporter = importer })
+
+	t.Cleanup(func() {
+		globalImporterOnce = sync.Once{}
+		globalImporter = nil
+		globalImporterErr = nil
+	})
+}
+
 func initGitRepo(t *testing.T, files map[string]string) string {
 	t.Helper()
 
@@ -528,14 +550,7 @@ func TestRemoteImporter_ResolveRemoteImport_GlobalImporter(t *testing.T) {
 	}))
 	defer server.Close()
 
-	globalImporterOnce = sync.Once{}
-	globalImporter = newTestRemoteImporter(t, &schema.AtmosConfiguration{})
-	globalImporterErr = nil
-	t.Cleanup(func() {
-		globalImporterOnce = sync.Once{}
-		globalImporter = nil
-		globalImporterErr = nil
-	})
+	useTestGlobalImporter(t, &schema.AtmosConfiguration{})
 
 	uri := server.URL + "/config.yaml"
 	matches, err := ResolveRemoteImport(&schema.AtmosConfiguration{}, uri)
@@ -556,14 +571,7 @@ func TestRemoteImporter_DownloadRemoteImport_GlobalImporter(t *testing.T) {
 	}))
 	defer server.Close()
 
-	globalImporterOnce = sync.Once{}
-	globalImporter = newTestRemoteImporter(t, &schema.AtmosConfiguration{})
-	globalImporterErr = nil
-	t.Cleanup(func() {
-		globalImporterOnce = sync.Once{}
-		globalImporter = nil
-		globalImporterErr = nil
-	})
+	useTestGlobalImporter(t, &schema.AtmosConfiguration{})
 
 	path, err := DownloadRemoteImport(&schema.AtmosConfiguration{}, server.URL+"/config.yaml")
 	require.NoError(t, err)
@@ -636,12 +644,11 @@ func TestProcessImportPath_Remote(t *testing.T) {
 	}))
 	defer server.Close()
 
-	// Reset the global importer for this test.
-	globalImporterOnce = sync.Once{}
-	globalImporter = nil
-	globalImporterErr = nil
-
 	atmosConfig := &schema.AtmosConfiguration{}
+	// Isolate the global importer's cache to a temp dir so the remote download does not
+	// write to (and flake on) the shared real user cache dir.
+	useTestGlobalImporter(t, atmosConfig)
+
 	basePath := filepath.Join(string(os.PathSeparator), "stacks")
 
 	// Process a remote import path.
@@ -694,12 +701,11 @@ func TestResolveImportPaths_MixedPaths(t *testing.T) {
 	}))
 	defer server.Close()
 
-	// Reset the global importer for this test.
-	globalImporterOnce = sync.Once{}
-	globalImporter = nil
-	globalImporterErr = nil
-
 	atmosConfig := &schema.AtmosConfiguration{}
+	// Isolate the global importer's cache to a temp dir so the remote download does not
+	// write to (and flake on) the shared real user cache dir.
+	useTestGlobalImporter(t, atmosConfig)
+
 	basePath := filepath.Join(string(os.PathSeparator), "stacks")
 
 	importPaths := []string{
@@ -830,12 +836,11 @@ func TestResolveImportPaths_ErrorPropagation(t *testing.T) {
 	}))
 	defer server.Close()
 
-	// Reset the global importer for this test.
-	globalImporterOnce = sync.Once{}
-	globalImporter = nil
-	globalImporterErr = nil
-
 	atmosConfig := &schema.AtmosConfiguration{}
+	// Isolate the global importer's cache to a temp dir so the remote download does not
+	// write to (and flake on) the shared real user cache dir.
+	useTestGlobalImporter(t, atmosConfig)
+
 	basePath := filepath.Join(string(os.PathSeparator), "stacks")
 
 	importPaths := []string{