fix(storage): prevent indefinite hang when calling Close() on async appendable uploads

[kalragauri]

Currently, when closing an appendable upload, the SDK delegates a kFlushAndClose signal to the underlying gRPC stream. However, in writer_connection_impl.cc, the caller was invoking Finish() immediately after flushing the write, failing to consume the server's final BidiWriteObjectResponse which effectively hangs the stream on the completion queue. This PR fixes the behavior to properly abide by the gRPC stream contract.

[gemini-code-assist]

Code Review

This pull request updates AsyncWriterConnectionImpl::OnClose to perform an asynchronous Read and OnQuery check before finishing the connection, and updates the corresponding unit and integration tests. A critical lifetime issue was identified in the new asynchronous chain, where capturing this by raw pointer in the .then() continuations could lead to a use-after-free if the connection object is destroyed before the chain completes. It is recommended to capture a shared pointer via shared_from_this() to extend the lifetime of the connection object.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.26%. Comparing base (ce70c7d) to head (4dd66b9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16255      +/-   ##
==========================================
- Coverage   92.26%   92.26%   -0.01%     
==========================================
  Files        2214     2214              
  Lines      206307   206326      +19     
==========================================
+ Hits       190355   190371      +16     
- Misses      15952    15955       +3     

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