fix(storage): guard against overlapping stream requests in AsyncWriterConnectionBuffered

[kalragauri]

In writer_connection_buffered.cc, WriteLoop previously computed its active state using:

writing_ = write_offset_ < resend_buffer_.size();

When an explicit Flush() was called with an empty/0-byte payload, resend_buffer_ had no unsent bytes, causing writing_ to evaluate to false. While FlushStep() was asynchronously executing on the gRPC stream, a subsequent operation (such as Finalize()) observed writing_ == false, bypassing the concurrency guard and initiating a concurrent gRPC operation on the same stream. This stream collision corrupted the gRPC state machine, causing pending promises to stall until timeout and resulting in premature teardown.

In this PR, WriteLoop has been updated to maintain writing_ = true across all dispatched asynchronous steps (WriteStep, FinalizeStep, CloseStep, FlushStep), clearing writing_ = false only when no operations are pending:

[gemini-code-assist]

Code Review

This pull request refactors the WriteLoop state machine in AsyncWriterConnectionBufferedState to manage the writing_ flag within individual execution branches, ensuring that the asynchronous pipeline is correctly marked as idle only when no further operations are pending. It also adds unit tests to verify that Finalize and Write operations are correctly queued and not executed concurrently while a Flush is in-flight. Feedback suggests simplifying WriteLoop by setting writing_ = true at the start of the function and only resetting it to false at the end if no asynchronous step is dispatched, which reduces duplicated code and aligns with the repository style guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.26%. Comparing base (03e43d3) to head (6e0db8f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16364      +/-   ##
==========================================
- Coverage   92.26%   92.26%   -0.01%     
==========================================
  Files        2237     2237              
  Lines      210434   210497      +63     
==========================================
+ Hits       194166   194207      +41     
- Misses      16268    16290      +22     

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