feat: implement AsyncMultiRangeDownloader with multiplexed bidi-gRPC stream support

[zhixiangli]

This PR implements AsyncMultiRangeDownloader with a new _StreamMultiplexer, enabling multiple concurrent range downloads to share a single bidirectional gRPC stream.

Before vs. After

Feature	Before	After (This PR)
Concurrency	Sequential or multiple connections	Concurrent over one connection
Overhead	High (multiple gRPC streams)	Low (multiplexed single stream)
Reliability	Per-stream retry logic	Unified generation-gated reopening

How it works

The system uses a background _StreamMultiplexer to manage the shared bidirectional stream:

1. Requests: Concurrent tasks send range requests (BidiReadObjectRequest) directly to the shared stream.
2. Multiplexing: A background Recv Loop listens for all responses. It uses the read_id in each response to route data to the correct task-specific asyncio.Queue.
3. Error Handling: If the stream breaks, a generation-gated lock ensures the stream is reopened only once. All active tasks receive a _StreamError and automatically retry using the new stream generation.

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Key Changes:

• _StreamMultiplexer: Background receiver loop for routing responses.
• Generation-Gated Reopening: Coordinates stream recovery across concurrent tasks.
• AsyncMultiRangeDownloader Integration: Full support for concurrent download_ranges calls.

[gemini-code-assist]

Code Review

This pull request introduces a _StreamMultiplexer to handle concurrent download tasks over a single bidirectional gRPC stream, replacing the previous locking mechanism in AsyncMultiRangeDownloader. The multiplexer routes responses to per-task asyncio queues based on read_id, allowing for better resource utilization. The lock parameter in download_ranges is now deprecated. Feedback focuses on improving the multiplexer's reliability and performance, specifically by using asyncio.gather to prevent head-of-line blocking during response broadcasting, ensuring the background receive loop terminates when no tasks are active, and adding error logging for observability.