fix(storage): preserve flush state across concurrent flushes in async writers

[kalragauri]

In SetFlushed(), both AsyncWriterConnectionBufferedState and AsyncWriterConnectionResumedState were unconditionally resetting the internal state flag flush_ = false at the very start of the method when a flush succeeded. If a caller issued multiple concurrent Flush() requests (or queued a new Flush() while a previous Flush() was in-flight), resetting flush_ = false prematurely caused the background WriteLoop to treat any remaining queued data as buffered data rather than explicit flush requests.

This PR modifies SetFlushed() to defer resetting flush_ = false until pending_flush_promises_.empty() evaluates to true (or when handling empty promise edge cases). This guarantees that as long as there are pending flush promises in the queue, flush_ remains true, instructing the WriteLoop to continue invoking flush operations on subsequent chunks until all queued flush promises are satisfied.

[gemini-code-assist]

Code Review

This pull request modifies the SetFlushed method in both AsyncWriterConnectionBufferedState and AsyncWriterConnectionResumedState to only reset the flush_ flag to false when there are no remaining pending flush promises. The unit tests have been updated to align with this change, expecting a second flush and query sequence. I have no feedback to provide as there are no review comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.29%. Comparing base (a9d896c) to head (1586e25).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16274   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files        2221     2221           
  Lines      207372   207384   +12     
=======================================
+ Hits       191386   191399   +13     
+ Misses      15986    15985    -1     

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