fix(storage): Implement clean half-close stream teardown for appendable uploads

[kalragauri]

Currently, closing an active write stream triggers a standard Flush() and cancels the stream. By using a clean half-close, the client abandons the write side cleanly, allowing GCS to persist all outstanding data.

This first PR introduces the core API interface, and the base stream close implementation. Subclass decorators (such as the Buffered, Resumed, and Tracing connections) are configured with a default fallback to standard Flush() to keep this PR small and safe. Follow-up PRs will customize the implementation under these sub-classes.

[gemini-code-assist]

Code Review

This pull request introduces a Close method to the AsyncWriterConnection interface and its implementation, allowing for a clean flush of pending data followed by a stream half-close. The changes update the PartialUpload state machine to support a new kFlushAndClose action and include comprehensive unit tests. Feedback indicates that the Close() implementation in AsyncWriterConnectionImpl might return a satisfied future before the underlying gRPC Finish() call completes, which could lead to missed errors or incomplete teardowns.

[codecov]

Codecov Report

❌ Patch coverage is 95.34884% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.71%. Comparing base (a899f46) to head (63fd0eb).

Files with missing lines	Patch %	Lines
google/cloud/storage/async/writer_connection.h	0.00%	2 Missing ⚠️
...d/storage/internal/async/writer_connection_impl.cc	94.73%	1 Missing ⚠️
...rage/internal/async/writer_connection_impl_test.cc	98.18%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16112      +/-   ##
==========================================
- Coverage   92.71%   92.71%   -0.01%     
==========================================
  Files        2353     2353              
  Lines      218986   219067      +81     
==========================================
+ Hits       203031   203100      +69     
- Misses      15955    15967      +12     

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