refactor(gax): consolidate grpc-rust test mocks and fixtures

[joshuatants]

For #5991

[gemini-code-assist]

Code Review

This pull request refactors the grpc_rust unit tests by consolidating duplicate mock streams, test messages, and helper structures (such as MockSendStream, MockRecvStream, and MockInvoker) into a new shared testing module. Feedback suggests further improving these shared test helpers by adding getter methods to MockSendStream and MockInvoker to allow direct inspection of captured messages, options, and headers, thereby reducing boilerplate in the test suites.

[codecov]

Codecov Report

❌ Patch coverage is 91.13924% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.39%. Comparing base (1506fc1) to head (d4190af).

Files with missing lines	Patch %	Lines
src/gax-internal/src/grpc/grpc_rust/testing.rs	84.44%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           grpc_rust    #6481   +/-   ##
==========================================
  Coverage      96.39%   96.39%           
==========================================
  Files            302      303    +1     
  Lines          85747    85734   -13     
==========================================
- Hits           82656    82647    -9     
+ Misses          3091     3087    -4     

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