fix(core): correct nested stack metadata collection boundary

[Ashutosh0x]

This PR fixes an issue where metadata from nested stacks was not being correctly collected or was leaking into the parent stack's artifact.

Problem

The collectStackMetadata function was traversing into nested stacks, causing incorrect metadata association and potential duplication in the cloud assembly.

Solution

Corrects the collectStackMetadata traversal to respect stack boundaries, ensuring metadata is associated with the correct stack artifact.

Split from #36871

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

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[Ashutosh0x]

Exemption Request

I have added a unit test metadata from nested stack is not collected in parent stack in
ew-style-synthesis.test.ts to verify the fix. This change affects internal metadata collection logic during synthesis and is best verified via unit tests checking the cloud assembly manifest, rather than a full integration test snapshot.