Fix ResponseCompression tests to work with different zlib implementations

[medhatiwari]

Fix ResponseCompression tests for system zlib compatibility

Fixes #63969

Changes

• Replace exact byte count assertions with functional decompression verification in CheckResponseCompressed
• Add CheckCompressionFunctionality method to verify compression by decompressing content
• Update HTTPS test methods to use appropriate verification based on compression state
• Handle edge cases: HEAD requests, NoCompression, and SendFileAsync scenarios

Testing

All existing tests pass with both zlib-ng and system zlib implementations.

cc: @giritrivedi

[joperezr]

Thanks a lot for the contribution @medhatiwari! This is a great change and allows us to run our tests against multiple zlib implementations! Ideally we'd want to run the tests in CI against both of those to have coverage and detect regressions but this is a good first step. The tradeoff here of losing brittleness for flexibility feels worth it, especially due to the improvement in correctness coverage.

cc: @BrennanConroy @akoeplinger any other thoughts here or can we go ahead and merge this in?

[BrennanConroy]

The general idea is good.