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

[kalragauri]

This is a follow-up to #16112. It implements the Close() API in AsyncWriterConnectionResumed and its internal state manager AsyncWriterConnectionResumedState. This allows resumed async uploads to perform a clean half-close.

This PR is similar to the Close implementation for the buffered writer in #16145.

[gemini-code-assist]

Code Review

This pull request implements the Close functionality for AsyncWriterConnectionResumed, adding the necessary state tracking, step functions, and resume handling, along with comprehensive unit tests. The review feedback suggests optimizing performance by passing Status objects by value and moving them in OnClose and SetClosed to avoid unnecessary copies, and correcting the misspelled test suite name WriteConnectionResumed to WriterConnectionResumed in the new tests.

[codecov]

Codecov Report

❌ Patch coverage is 95.92593% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (ce5aa34) to head (92a06ce).

Files with missing lines	Patch %	Lines
...torage/internal/async/writer_connection_resumed.cc	88.54%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16163    +/-   ##
========================================
  Coverage   92.20%   92.21%            
========================================
  Files        2264     2264            
  Lines      209092   209324   +232     
========================================
+ Hits       192802   193028   +226     
- Misses      16290    16296     +6     

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