refactor(gax): consolidate grpc-rust test mocks

[joshuatants]

For #5991.

[gemini-code-assist]

Code Review

This pull request refactors the gRPC test helpers by consolidating common mock implementations—such as MockSendStream, MockRecvStream, MockInvoker, and FailAfterFirstSendStream—from bidi.rs and unary.rs into a shared testing.rs module, reducing code duplication. The review feedback suggests simplifying MockInvoker by removing an unnecessary Arc wrapper around observed_headers and using TestMessage::new in bidi.rs to maintain consistency across tests.

[codecov]

Codecov Report

❌ Patch coverage is 98.18182% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.42%. Comparing base (bf6cba4) to head (041d709).
⚠️ Report is 1 commits behind head on grpc_rust.

Files with missing lines	Patch %	Lines
src/gax-internal/src/grpc/grpc_rust/testing.rs	97.14%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           grpc_rust    #6519    +/-   ##
===========================================
  Coverage      96.42%   96.42%            
===========================================
  Files            307      307            
  Lines          86490    86331   -159     
===========================================
- Hits           83397    83245   -152     
+ Misses          3093     3086     -7     

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