fix(defaultStackSynthesizer): prevent App level synthesizer from propagating assets across stacks

[castodius]

Issue 29071
Closes #29071

Reason for this change
The method reusableBind on DefaultStackSynthesizer which is intended to copy the current DefaultStackSynthesizer ends up returning an instance with a reference to the AssetManifestBuilder instance. All stacks end up sharing the same asset manager and additional assets end up in the wrong stacks.

Description of changes
The method reusableBind now instantiates a new AssetManifestBuild eliminating the problem.

Description of how you validated changes
I have added a unit test which fails if my changes are not included. In addition I took the questionable cowboy approach of verifying the code change by changing minified code for default-stack-synthesizer.ts in a test project.

Checklist
[ x ] My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

[castodius]

I was not sure where to place the unit test, so I simply placed it under the only describe available. Perhaps it belongs in a new describe together with this test specifically for "app level synthesizer".

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 3388e94
• Result: SUCCEEDED
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