feat(storage): Add full object checksum validation when resuming async bidi uploads

[v-pratap]

When taking over or resuming an async bi-directional upload (resumable or appendable) via QueryWriteStatus or MakeAppendableWriter, if the server provides persisted_data_checksums, seed the client-side CRC32C hash function from the acknowledged persisted state and forward the checksums directly to AsyncWriterConnectionImpl. This enables complete end-to-end full object integrity verification upon finalization.

[gemini-code-assist]

Code Review

This pull request adds support for resuming unbuffered uploads with persisted data checksums in the asynchronous GCS client. When resuming, if CRC32C validation is enabled and the server provides persisted checksums, the hash function is initialized with the existing CRC32C value and size. Feedback highlights a critical bug in MakeAppendableWriter where a fallback to CreateNullHashFunction is needed when persisted_size > 0 but validation cannot be seeded, preventing incorrect checksum calculations. Additionally, the new unit test should be enhanced to explicitly assert that checksums are not sent when validation is disabled.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.71%. Comparing base (8d79f18) to head (d2dec7f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16138      +/-   ##
==========================================
- Coverage   92.72%   92.71%   -0.01%     
==========================================
  Files        2356     2356              
  Lines      220517   220617     +100     
==========================================
+ Hits       204468   204543      +75     
- Misses      16049    16074      +25     

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