feat(storage): support pre-warming read ranges in AsyncConnection

[kalragauri]

This PR implements the backend plumbing and happy-path caching to support pre-warming read ranges in the GCS async client.

Key changes:

• Introduced ReadRangesOption containing a list of ReadRangeConfig (offset and length) to configure which ranges to pre-warm.
• Updated AsyncConnectionImpl::Open to populate the initial BidiReadObjectRequest with the configured read_ranges.
• Updated ObjectDescriptorImpl constructor to initialize ReadRange objects for pre-warmed ranges and register them on the initial stream so incoming GCS data chunks are routed correctly and not discarded.
• Implemented a prewarmed_ranges_ cache in ObjectDescriptorImpl. When the user calls Read(), the cache is checked. On a hit, the pre-warmed ReadRange is claimed and returned as an ObjectDescriptorReader.

[gemini-code-assist]

Code Review

This pull request introduces support for pre-warmed ranges in the asynchronous GCS client, allowing specified ranges to be downloaded as soon as a stream opens. It adds the ReadRangesOption configuration, updates AsyncConnectionImpl::Open to populate the initial request with these ranges, and modifies ObjectDescriptorImpl to cache and claim them during reads. The feedback suggests critical improvements to the caching mechanism: using std::multimap instead of std::map to prevent resource leaks when duplicate ranges are configured, using emplace for insertion, and utilizing std::move to avoid unnecessary copies of std::shared_ptr when claiming cached ranges.

[codecov]

Codecov Report

❌ Patch coverage is 99.28058% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.31%. Comparing base (6f13a6b) to head (b21c151).

Files with missing lines	Patch %	Lines
...torage/internal/async/connection_impl_open_test.cc	98.90%	1 Missing ⚠️
...d/storage/internal/async/object_descriptor_impl.cc	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16275      +/-   ##
==========================================
+ Coverage   92.29%   92.31%   +0.01%     
==========================================
  Files        2226     2226              
  Lines      208573   208851     +278     
==========================================
+ Hits       192504   192792     +288     
+ Misses      16069    16059      -10     

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[kalragauri]

/gcbrun