fix(storage): remove duplicate Query calls from AsyncWriterConnectionBuffered and AsyncWriterConnectionResumed

[kalragauri]

Following the merge of PR #16307, Flush() now guarantees that server state (persisted_size) is fully updated before satisfying its future. Since Flush() handles network synchronization natively, higher-level wrappers (AsyncWriterConnectionBuffered and AsyncWriterConnectionResumed) no longer need to trigger redundant Query() calls to update their local tracking after a flush. This PR removes those duplicated Query() network calls and cleans up the resulting callback paths.

[gemini-code-assist]

Code Review

This pull request refactors both the buffered and resumed async writer connections to retrieve the persisted size directly from the connection's persisted state instead of making an asynchronous query call, and updates the corresponding tests. The review feedback identifies three opportunities to avoid unnecessary copies of non-fundamental types, specifically recommending the use of const references for the persisted state and moving the status result when setting the flushed state.

[codecov]

Codecov Report

❌ Patch coverage is 98.84393% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (441b41f) to head (f132f31).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...orage/internal/async/writer_connection_buffered.cc	83.33%	1 Missing ⚠️
...torage/internal/async/writer_connection_resumed.cc	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16315      +/-   ##
==========================================
- Coverage   92.24%   92.23%   -0.01%     
==========================================
  Files        2227     2227              
  Lines      209234   209169      -65     
==========================================
- Hits       193008   192934      -74     
- Misses      16226    16235       +9     

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