fix(storage): add Close() support to AsyncWriterConnectionBuffered

[kalragauri]

This is a follow-up to #16112. It implements the Close() API in AsyncWriterConnectionBuffered and its internal state manager AsyncWriterConnectionBufferedState. This allows buffered async uploads to perform a clean half-close after ensuring all previously buffered data is fully flushed.

[gemini-code-assist]

Code Review

This pull request implements the Close functionality for AsyncWriterConnectionBuffered and its internal state, including handling close steps, state resumption on failures, and adding corresponding unit tests. The review feedback highlights two issues: first, a critical bug where CloseStep may be triggered before the resend_buffer_ is fully drained, leading to data loss; second, a potential issue where subsequent calls to Close() return an invalid future because closed_future_ is moved on the first invocation.

[codecov]

Codecov Report

❌ Patch coverage is 99.57447% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.20%. Comparing base (fd73bee) to head (9d1a49e).

Files with missing lines	Patch %	Lines
...orage/internal/async/writer_connection_buffered.cc	98.63%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16145    +/-   ##
========================================
  Coverage   92.20%   92.20%            
========================================
  Files        2264     2264            
  Lines      208864   209084   +220     
========================================
+ Hits       192579   192786   +207     
- Misses      16285    16298    +13     

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[bajajneha27]

/gcbrun