fix(storage): hedged read attempts must not own the connection

[kalragauri]

This PR re-enables RetryClientTest.HedgedReadRecordsMetricsOnGlobalMeterProvider (skipped in #16523) and fixes the lifetime bug behind the gmock leak it reported.

Two references allowed a pool worker to hold the last reference to StorageConnectionImpl after Read() had already returned, destroying the connection (and its mock stub and metrics instruments) on a detached worker
thread after main()'s leak check:

• HedgedObjectReadSource::ReadRaced() captured child_factory_ by shared_ptr in each attempt task, and the factory captures the connection. Because a worker destroys its task only after promise.set_value() unblocks the caller, the winning worker kept the connection alive past the end of the read. Attempts now capture a weak_ptr and lock it only for the open call; an attempt that starts after the source is destroyed retires with kCancelled.
• A losing attempt owns a RetryObjectReadSource (which holds the connection) until its Close() returns. This is unchanged: the test now issues one more synchronous read on the single read worker after unblocking the loser, which (FIFO) cannot complete until the loser's task has been destroyed, so the test thread holds the last connection reference. Dropping a Client mid-close in production still tears the connection down on a detached worker, same as AutomaticallyCreatedBackgroundThreads; that is pre-existing and out of scope for this PR.

Fixes #16522

[gemini-code-assist]

Code Review

This pull request refactors HedgedObjectReadSource to pass a std::weak_ptr of the ChildFactory to racing attempts, preventing them from keeping the factory and its associated connection/thread pools alive after the read source is destroyed. Additionally, it re-enables the previously skipped HedgedReadRecordsMetricsOnGlobalMeterProvider test with updated synchronization logic and adds a new unit test QueuedAttemptAfterDestructionDoesNotOpen to verify the lifetime behavior. I have no feedback to provide as there are no review comments to assess.

[codecov]

Codecov Report

❌ Patch coverage is 91.39785% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.36%. Comparing base (54869ea) to head (e725508).

Files with missing lines	Patch %	Lines
...storage/internal/hedged_object_read_source_test.cc	79.41%	7 Missing ⚠️
...loud/storage/internal/hedged_object_read_source.cc	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16534      +/-   ##
==========================================
+ Coverage   92.34%   92.36%   +0.01%     
==========================================
  Files        2262     2262              
  Lines      217173   217309     +136     
==========================================
+ Hits       200549   200708     +159     
+ Misses      16624    16601      -23     

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