fix(storage): Resolve potential race condition in AsyncWriterConnectionImpl

[kalragauri]

This change addresses a potential race condition in AsyncWriterConnectionImpl between the destructor and asynchronous callback operations.

The impl_ member variable could be moved in the destructor while simultaneously being accessed by a callback function (e.g., within OnPartialUpload which can call impl_->Finish()). This concurrent access could lead to a use-after-free error under certain timing conditions. To resolve this, this commit introduces a std::mutex to protect accesses to the impl_ member.

Why other methods (Write, Flush, etc.) are not protected:

The library's public API contract forbids users from calling methods like Write(), Flush(), Query(), or Finalize() while destruction is in progress or while another operation is already pending. As long as the user follows this contract, these methods will never run concurrently with the destructor.

The only reason we need protection in Finish() and OnFinalUpload() is because they are called by the library's own internal asynchronous callbacks (running on the Completion Queue thread), which can execute concurrently with the destructor (running on the user's thread) if an operation completes or fails just as the object is being destroyed.

[gemini-code-assist]

Code Review

This pull request introduces a mutex to protect the impl_ member variable during destruction and within the Finish method of AsyncWriterConnectionImpl to prevent data races. While these changes improve thread safety, the reviewer noted that the fix is incomplete as other methods accessing impl_ and shared state like latest_write_handle_ remain unprotected, potentially leading to race conditions or null pointer dereferences.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.68%. Comparing base (e5d585c) to head (0e5e0c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16099      +/-   ##
==========================================
- Coverage   92.69%   92.68%   -0.01%     
==========================================
  Files        2353     2353              
  Lines      218328   218339      +11     
==========================================
- Hits       202386   202375      -11     
- Misses      15942    15964      +22     

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