fix(storage): support download stall timeout in async BiDi reads

[kalragauri]

When opening an async bidirectional read stream (AsyncBidiReadObject), a network stall or silent disconnect during the initial stream handshake (Start, Write, or initial Read) can cause the call to hang indefinitely.

This PR does the following:

• Wraps AsyncBidiReadObject streaming RPC with AsyncStreamingReadWriteRpcTimeout in OpenObject::Call().
• Extracts timeout configuration from DownloadStallTimeoutOption or TransferStallTimeoutOption, setting the expected chunk size to ServiceConstants::MAX_READ_CHUNK_BYTES (2 MiB).
• Adds unit tests in open_object_test.cc verifying watchdog timer scheduling, reset on progress, and stream cancellation on timeout.

[gemini-code-assist]

Code Review

This pull request introduces timeout handling to the asynchronous OpenObject RPC stream by wrapping the underlying RPC with AsyncStreamingReadWriteRpcTimeout using a scaled stall timeout, and adds a corresponding unit test to verify timeout cancellation. The review feedback suggests replacing several usages of auto with explicit types (for timeout, rpc, pending, and response) to adhere to the repository's style guide against obscuring domain and return types.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.28%. Comparing base (9230136) to head (78ea5f8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16418   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files        2246     2246           
  Lines      212584   212648   +64     
=======================================
+ Hits       196175   196237   +62     
- Misses      16409    16411    +2     

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