impl(gaxi): add unimplemented stub helper for bidi

[suzmue]

We will use this in the generated code for the unimplemented stub to replace the workaround that uses the existing unimplemented_stub.

[gemini-code-assist]

Code Review

This pull request adds support for an unimplemented bidirectional streaming stub (unimplemented_bidi_stub) in src/gax-internal/src/unimplemented.rs, conditionally compiled under the google_cloud_unstable_gapic_streaming feature flag. It also introduces corresponding unit tests to verify that both the standard and bidirectional unimplemented stubs panic as expected. I have no feedback to provide on these changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.27%. Comparing base (2a3e1e0) to head (446b5a1).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6319   +/-   ##
=======================================
  Coverage   96.26%   96.27%           
=======================================
  Files         283      283           
  Lines       73110    73119    +9     
=======================================
+ Hits        70380    70393   +13     
+ Misses       2730     2726    -4     

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