[enhancement] aws_neptune_cluster: add Point in time restore feature

[sukumaar]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

Adds restore_to_point_in_time support to aws_neptune_cluster, enabling users to create point-in-time restored clusters and fast clones via the Neptune RestoreDBClusterToPointInTime API.

Previously, users who wanted to clone a Neptune cluster or restore to a specific point in time had to do so manually via the AWS console/CLI and then import the resource. This change adds a restore_to_point_in_time configuration block that supports:

• source_cluster_identifier to specify which cluster to restore from
• restore_type for choosing between full-copy or copy-on-write (fast clone)
• restore_to_time to restore to a specific UTC timestamp
• use_latest_restorable_time to restore to the most recent available backup point

The implementation follows the same pattern already established by aws_rds_cluster PITR support, adapted for Neptune's API surface.

Relations

Closes #44620

References

Output from Acceptance Testing

% make testacc TESTS=TestAccNeptuneCluster_restoreToPointInTime PKG=neptune ACCTEST_PARALLELISM=1 ACCTEST_TIMEOUT=4880m
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
make: Validating schemas
ok      github.com/hashicorp/terraform-provider-aws/internal/provider/sdkv2     (cached)
ok      github.com/hashicorp/terraform-provider-aws/internal/provider/framework (cached)
make: Running acceptance tests on branch: 🌿 neptune-pitr 🌿...
TF_ACC=1 go1.26.4 test ./internal/service/neptune/... -v -count 1 -parallel 1 -run='TestAccNeptuneCluster_restoreToPointInTime'  -timeout 4880m -vet=off -buildvcs=false
2026/06/19 11:13:19 Creating Terraform AWS Provider (SDKv2-style)...
2026/06/19 11:13:19 Initializing Terraform AWS Provider (SDKv2-style)...
=== RUN   TestAccNeptuneCluster_restoreToPointInTime_latestRestorableTime
=== PAUSE TestAccNeptuneCluster_restoreToPointInTime_latestRestorableTime
=== RUN   TestAccNeptuneCluster_restoreToPointInTime_copyOnWrite
=== PAUSE TestAccNeptuneCluster_restoreToPointInTime_copyOnWrite
=== CONT  TestAccNeptuneCluster_restoreToPointInTime_latestRestorableTime
--- PASS: TestAccNeptuneCluster_restoreToPointInTime_latestRestorableTime (1332.22s)
=== CONT  TestAccNeptuneCluster_restoreToPointInTime_copyOnWrite
--- PASS: TestAccNeptuneCluster_restoreToPointInTime_copyOnWrite (1463.28s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/neptune    2800.882s

[github-actions]

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