refactor(gax-internal): extract helpers and abort on receiver drop

[suzmue]

Move gRPC streaming request and response transformations into a dedicated internal streaming module with unit test coverage.

Abort the in-flight bidi stream connection attempt immediately when the ResponseReceiver is dropped before the stream is established.

Remove DEFAULT_REQUEST_CHANNEL_CAPACITY from public options::internal.

[gemini-code-assist]

Code Review

This pull request refactors gRPC streaming helpers by moving them into a dedicated internal streaming module in src/gax-internal/src/grpc/streaming.rs and adding comprehensive unit tests. The feedback suggests improving code style in the test mocks by removing unnecessary else blocks to keep the logic flow linear, and provides guidance on how to implement tracing span propagation for the spawned task in spawn_bidi_stream.

[codecov]

Codecov Report

❌ Patch coverage is 99.13793% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.37%. Comparing base (b1fab5f) to head (91c1a61).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gax-internal/src/grpc.rs	91.42%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6473      +/-   ##
==========================================
+ Coverage   96.36%   96.37%   +0.01%     
==========================================
  Files         295      296       +1     
  Lines       83762    84073     +311     
==========================================
+ Hits        80716    81025     +309     
- Misses       3046     3048       +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.