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

[dotnet-policy-service]

Thanks for your PR, @@medhatiwari. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.