fix(codepipeline): prevent S3 bucket orphaning in cross-region support stacks

[ychjamie]

Issue # (if applicable)

Closes #35654.

Reason for this change

When using CDK Pipelines with cross-region deployments, the S3 replication bucket created by the cross-region support stack is orphaned during cdk destroy operations. This prevents clean teardown of preview/ephemeral environments and causes redeployment failures due to existing bucket conflicts.

Description of changes

Added removalPolicy: cdk.RemovalPolicy.DESTROY to the S3 replication bucket in CrossRegionSupportConstruct. This aligns the bucket's lifecycle management with the existing KMS key pattern in the same file and enables proper cleanup during stack destruction.

Key changes:

• Added removalPolicy: cdk.RemovalPolicy.DESTROY to S3 bucket configuration (line 85 in cross-region-support-stack.ts)
• Added 2 unit tests to verify CloudFormation template includes DeletionPolicy: Delete
• Tests validate lifecycle consistency across all synthesis modes

Design decision: Used removalPolicy only (not autoDeleteObjects) to avoid bootstrap asset dependencies and ensure compatibility with all synthesis modes, including BootstraplessSynthesizer.

CloudFormation impact: Adds DeletionPolicy: Delete to the S3 bucket resource in cross-region support stacks. Existing stacks can be updated without issues.

Describe any new or updated permissions being added

N/A - No IAM permissions changes. This change only affects CloudFormation resource deletion behavior.

Description of how you validated changes

• Unit tests: Added 2 new test cases (8 total executions via parameterization):
  • "support stack bucket has DeletionPolicy Delete" - Verifies CloudFormation template includes DeletionPolicy: Delete
  • "bucket has DeletionPolicy Delete matching KMS key pattern" - Ensures lifecycle consistency
• Test coverage: All synthesis modes tested (legacy/modern × with/without Stage)
• Regression testing: All 366 existing tests continue to pass
• CloudFormation validation: Template generation verified to include DeletionPolicy: Delete on S3 bucket resource

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

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