fix(assertions): nested stacks inside non-root stages don't resolve templates

[Styerp]

Templates are placed inside a path described by their closest assembly. Using this assembly lets nested stack templates resolve, regardless of their stage depth. Previously, they would fail to resolve if they were in a stage other than the app.

Closes #24004

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[gitpod-io]

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[Styerp]

The pull request linter fails with the following errors:

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This is in the library for testing CDK -> CFN transformations. I don't think there's an integration test here. Happy to update if there's one somewhere I'm not seeing.

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[aws-cdk-automation]

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