feat(storage): implement 2 MiB coalescing buffer for appendable upload

[vsharonlynn]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a CoalescingBuffer to batch client append operations into standard 2 MiB chunks, matching the GCS protocol limit, and integrates it into the AppendableObjectWriterTransport along with updated unit tests. The review feedback suggests optimizing the buffer's ingestion logic by using direct slicing to avoid atomic reference counting overhead, and refactoring the flush method to remove an unnecessary else block in compliance with the repository style guide.

[gemini-code-assist]

Slicing Bytes via chunk.slice(..needed) creates a new Bytes handle, which incurs atomic reference counting overhead. Since extend_from_slice only requires a byte slice (&[u8]), we can pass &chunk[..needed] directly to avoid this overhead. Please document this safety guarantee with a comment.

Suggested change

	let prefix = chunk.slice(..needed);
	self.buffer.extend_from_slice(&prefix);
	// SAFETY: `needed` is guaranteed to be within the bounds of `chunk`.
	self.buffer.extend_from_slice(&chunk[..needed]);

References

1. When slicing a collection directly in Rust, document the safety guarantee with a comment.

[gemini-code-assist]

According to the repository style guide, we should avoid unnecessary else blocks to keep the main logic flow linear and reduce indentation.

        if self.buffer.is_empty() {
            return None;
        }
        let residual = self.buffer.split().freeze();
        self.buffer.reserve(COALESCING_CHUNK_SIZE);
        Some(residual)

References

1. Avoid unnecessary else blocks to reduce indentation and keep the main logic flow linear. ^(link)

[codecov]

Codecov Report

❌ Patch coverage is 95.80838% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.51%. Comparing base (178dc27) to head (c133b14).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...torage/src/storage/bidi_write/coalescing_buffer.rs	95.86%	5 Missing ⚠️
src/storage/src/storage/bidi_write/transport.rs	95.65%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6571      +/-   ##
==========================================
- Coverage   96.52%   96.51%   -0.01%     
==========================================
  Files         304      305       +1     
  Lines       87969    88123     +154     
==========================================
+ Hits        84908    85054     +146     
- Misses       3061     3069       +8     

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