[WIP] Unit tests should catch unexpected jumps in file sizes in VSIX

[Copilot]

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Original issue description:

We have unit tests (PackageContentTests?) that validate the set of files in the VSIX files we build. These only catch if files are unexpectedly removed or added.

We should also have a test that fails if the size of included files jumps unexpectedly. It might also be helpful to check if file sizes drop unexpectedly too.

We would have to baseline the expected file sizes in the tests themselves. On occasion we would need to update these baselines due to product evolution.

Fixes #9773.

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