fix(storage): track persisted byte offset for async writer flush promises

[kalragauri]

In AsyncWriterConnectionBufferedState and AsyncWriterConnectionResumedState, SetFlushed() completed pending Flush() promises using a simple FIFO pop_front(). Because server-side persisted_size confirmations are cumulative byte offsets rather than 1:1 request/response tokens, a subsequent Flush() could be prematurely satisfied by an earlier acknowledgment while its buffered writes were still in-flight.

This PR does the following:

• Introduces a PendingFlush struct to pair each Flush() promise with its expected target_offset (buffer_offset_ + resend_buffer_.size()).
• Updates SetFlushed() to dequeue and satisfy only promises whose target_offset <= persisted_size.

[gemini-code-assist]

Code Review

This pull request updates both AsyncWriterConnectionBufferedState and AsyncWriterConnectionResumedState to track outstanding Flush() promises alongside their stream target offsets using a new PendingFlush struct. This ensures that multiple queued flush operations are only completed when the server's persisted size reaches or exceeds their respective target offsets. Unit and integration tests have been added to verify this behavior. Feedback is provided regarding a compilation error in writer_connection_resumed.cc where persisted_size (a StatusOr<std::int64_t>) is passed directly to SetFlushed which expects std::int64_t; it is recommended to explicitly check persisted_size.ok() before extracting the value.

[codecov]

Codecov Report

❌ Patch coverage is 98.60140% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.28%. Comparing base (4d385e8) to head (8c2fe77).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...torage/internal/async/writer_connection_resumed.cc	92.59%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16417      +/-   ##
==========================================
+ Coverage   92.26%   92.28%   +0.02%     
==========================================
  Files        2246     2246              
  Lines      212734   212774      +40     
==========================================
+ Hits       196273   196366      +93     
+ Misses      16461    16408      -53     

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