fix(request-sharing): correct clone, gc task lifetime, and drop zeroing for RequestSharing

[metalurgical]

Description

Correct Clone implementation for RequestSharing to share the in-flight cache instead of creating a new one, allow the GC task to exit when all handles are dropped, and fix Drop to only zero on the last handle.

Changes

• Change in_flight: self.in_flight.clone() instead of Default::default()
• spawn_gc modified to check Arc and exit once all handles are gone.
• Drop checks Arc::strong_count(&self.in_flight) == 1
• Added tests

How to test

cargo nextest run -p request-sharing

[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request updates the RequestSharing implementation to share the in-flight request cache across clones and modifies the garbage collection task to use a weak reference, ensuring the task terminates when all handles are dropped. A critical race condition was identified where the request_sharing_cached_items metric might remain at a non-zero value if the GC task holds a strong reference during the final drop of the cache handles. It is recommended to explicitly zero the metric within the GC task's exit path to ensure consistent state reporting.