fix(storage): Reset write offset on gRPC BidiWriteObject resumable uploads

[kalragauri]

When a stream breaks, the connection enters a resume state. A key step in OnResume is to create a new underlying connection. This new connection is initialized to start writing data from the persisted size reported by the GCS backend.

The existing logic did not work as expected in some cases because it used relative decrements that became invalid after buffer truncation. This could happen if the size of payload persisted by the server did not match the size sent by the client.

This change introduces an is_resume flag to the OnQuery method. When OnQuery is called as part of the Resume() flow (i.e., after a new underlying connection is established), write_offset_ is now explicitly set to 0. This ensures that the WriteLoop correctly starts sending from the beginning of the truncated resend_buffer_ on the resumed stream.

[gemini-code-assist]

Code Review

This pull request modifies the OnQuery method in both AsyncWriterConnectionBuffered and AsyncWriterConnectionResumed to include an is_resume flag, ensuring that write_offset_ is correctly reset to zero upon a resume. Corresponding unit tests have been added to verify this logic. The review feedback highlights a potential integer underflow risk when updating write_offset_ if the server's reported persisted size exceeds the client's current write position, suggesting a defensive check to reset the offset to zero in such scenarios.

[codecov]

Codecov Report

❌ Patch coverage is 98.57143% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.71%. Comparing base (1b9b3c1) to head (244a1e1).

Files with missing lines	Patch %	Lines
.../internal/async/writer_connection_buffered_test.cc	97.61%	1 Missing ⚠️
...torage/internal/async/writer_connection_resumed.cc	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16083    +/-   ##
========================================
  Coverage   92.70%   92.71%            
========================================
  Files        2351     2351            
  Lines      217954   218088   +134     
========================================
+ Hits       202047   202190   +143     
+ Misses      15907    15898     -9     

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