feat(storage): Extend idempotency token use to all async mutation operations

[kalragauri]

This change extends the use of idempotency tokens to all mutation operations within the asynchronous gRPC-based client library. A unique token is generated client-side and included in the request metadata under the x-goog-gcs-idempotency-token header.

[gemini-code-assist]

Code Review

This pull request standardizes the addition of idempotency tokens across asynchronous storage operations—specifically Insert, Compose, Delete, and Appendable Uploads—by utilizing a new AddIdempotencyToken helper function. The changes include updating the AsyncConnectionImpl to capture and apply invocation IDs and adding unit tests to verify token presence in gRPC metadata. Review feedback recommends using GoogleMock matchers to simplify metadata verification in tests and removing an unused variable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.68%. Comparing base (e5d585c) to head (2b7d63b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16100      +/-   ##
==========================================
- Coverage   92.69%   92.68%   -0.01%     
==========================================
  Files        2353     2353              
  Lines      218328   218377      +49     
==========================================
+ Hits       202386   202413      +27     
- Misses      15942    15964      +22     

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