impl(gax-internal): support server-side streaming

[suzmue]

Add the execute_server_streaming helper on the gRPC transport client to support server-side streaming RPCs. The helper handles converting domain requests to proto messages and decodes the response stream using decode_response_stream into a ResponseReceiver.

Gate server streaming execution helpers under the
google_cloud_unstable_gapic_streaming cfg.

Enable include_server_streaming_methods for Showcase in librarian.yaml, regenerate google-cloud-showcase-v1beta1, and add integration tests for server-side streaming.

For #2318

[gemini-code-assist]

Code Review

This pull request adds support for server-side streaming RPCs in the Rust SDK. It implements execute_server_streaming in the internal gRPC client, adds unimplemented server streaming stubs, and updates the generated showcase client, builders, and transport layers to support streaming methods like expand, stream_blurbs, and attempt_streaming_sequence. Comprehensive integration and unit tests are also included. The review feedback recommends using the fully qualified path google_cloud_gax::streaming::decode_response_stream in src/gax-internal/src/grpc.rs to ensure consistency with the fully qualified ResponseReceiver type used nearby.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.39%. Comparing base (e532748) to head (21d79f2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6504      +/-   ##
==========================================
- Coverage   96.39%   96.39%   -0.01%     
==========================================
  Files         301      301              
  Lines       84762    84800      +38     
==========================================
+ Hits        81710    81744      +34     
- Misses       3052     3056       +4     

☔ 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.