feat : use account‑regional suffix for default staging bucket

[naaa760]

Issue # (if applicable)

Closes #37245.

Reason for this change

• Default staging buckets were not using the S3 account‑regional namespace, leaving names open to squatting.

Description of changes

• Append -an to the default staging bucket name in DefaultStagingStack.
• Update unit tests to expect the new bucket name and URLs.
• Refresh integ snapshots to match the -an suffixed bucket names.

Describe any new or updated permissions being added

• None. No IAM roles or policies were added or changed.
• Description of how you validated changes
• Updated and ran the @aws-cdk/app-staging-synthesizer-alpha tests.
• Regenerated and reviewed affected integration snapshots.

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Features must contain a change to a README file.
❌ Features must contain a change to an integration test file and the resulting snapshot.
❌ The title prefix of this pull request must be one of "feat|fix|build|chore|ci|docs|style|refactor|perf|test|revert"

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[dehli]

Awesome! From the docs, it seems like there will be a new BucketNamespace property on the S3 bucket resource in CloudFormation. Does this change work without setting that property (which I don't think has landed yet).

Edit: confirmed this won't work due to the missing namespace on the bucket