feat(codepipeline-actions): expose PR-specific variables in CodeStarConnectionsSourceAction

[hmkim]

Summary

• Add sourceBranchName, destinationBranchName, pullRequestId, and pullRequestTitle to CodeStarSourceVariables interface and the variables getter
• These variables are available when a pipeline is triggered by a PR event but were previously only accessible via a private method workaround: (sourceAction as any)['variableExpression']('SourceBranchName')
• Non-breaking additive change (new readonly properties on an output interface)

Closes #31000

Test plan

• Added unit test verifying PR-specific variables resolve to correct #{NS.SourceBranchName} etc. expressions
• All existing tests continue to pass (9/9)

🤖 Generated with Claude Code

[aws-cdk-automation]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ✅	Skipped	Failed
Security Guardian Results	24 ran	24 passed

Test	Result
No test annotations available

• View Security Guardian Results

[github-actions]

⚠️ Experimental Feature: This security report is currently in experimental phase. Results may include false positives and the rules are being actively refined.
Please try merge from main to avoid findings unrelated to the PR.

---

	Tests	Passed ☑️	Skipped	Failed ❌️
Security Guardian Results with resolved templates	24 ran	23 passed		1 failed

Test	Result
Security Guardian Results with resolved templates
packages/@aws-cdk-testing/framework-integ/test/aws-codepipeline-actions/test/integ.pipeline-with-pipeline-triggers.js.snapshot/aws-cdk-codepipeline-with-pipeline-triggers.template.json
iam-role-root-principal-needs-conditions.guard	❌ failure

• View Security Guardian Results with resolved templates