test(showcase): add bidi streaming generated code and integration test

[suzmue]

Generate the prototyping showcase crate with bidi streaming methods enabled. The library code and its integration test are gated behind google_cloud_unstable_gapic_streaming.

For #2318

[suzmue]

If you look at the last commit, this will remove the clutter from updating the oneof conversions across the repo.

[gemini-code-assist]

Code Review

This pull request enables bidirectional streaming methods for the showcase service in the Rust SDK, introducing the chat and connect streaming methods across the client, stub, tracing, and transport layers. It also updates several generated conversion files (BigQuery Write, Firestore, Spanner, Storage) to box certain enum variants. One issue was identified in the chat integration test where a silent pass could occur if the stream is empty; it is recommended to explicitly assert that a response is received.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.25%. Comparing base (8fea054) to head (c4a3be9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6275   +/-   ##
=======================================
  Coverage   96.24%   96.25%           
=======================================
  Files         280      280           
  Lines       72194    72194           
=======================================
+ Hits        69484    69488    +4     
+ Misses       2710     2706    -4     

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[suzmue]

Converted to draft since some of the issues need to be addressed upstream first: googleapis/librarian#7176

[suzmue]

PR Status: On Hold

#6277 <- I want this to go in first so this PR doesn't get cluttered with shopping and bigquery changes.