fix(storage): consume server responses before finishing stream in AsyncWriter Close/Finalize

[kalragauri]

This PR fixes an issue where calling AsyncWriter::Close() or Finalize() hangs indefinitely when the GCS service returns intermediate response messages before closing the stream.

Under the hood, Close() sends a kFlushAndClose request, which prompts GCS to respond. Previously, AsyncWriterConnectionImpl called Finish() on the underlying gRPC stream immediately after writing without consuming any remaining server responses. This unread data on the stream caused Finish() to block waiting for response consumption, leading to a deadlock.

The PR also updates unit tests that mock the upload stream to expect the additional Read() call (returning EOF) and adjust the sequencer expectations.

[gemini-code-assist]

Code Review

This pull request refactors OnClose and OnFinalUpload in AsyncWriterConnectionImpl to use a recursive ConsumeLoop that reads and processes all intermediate server responses before finalizing the upload, preventing hangs when multiple responses are sent before EOF. Corresponding unit tests are updated and a new test is added to verify this behavior. The review feedback correctly identifies compilation errors in both refactored methods where the undefined variable self is referenced inside lambdas capturing [this], and provides code suggestions to resolve these issues by calling Finish() directly.

[codecov]

Codecov Report

❌ Patch coverage is 99.35760% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.29%. Comparing base (a9d896c) to head (85a4af4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/storage/internal/async/writer_connection_impl.cc	93.75%	2 Missing ⚠️
...rage/internal/async/writer_connection_impl_test.cc	99.48%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16270    +/-   ##
========================================
  Coverage   92.29%   92.29%            
========================================
  Files        2221     2221            
  Lines      207372   207619   +247     
========================================
+ Hits       191386   191629   +243     
- Misses      15986    15990     +4     

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