Cleaned up possible concurrent modification in SidecarRetriever.

[rolfyone]

fixes #10288

Copying the maps does avoid the exception and short of some crazy locking is probably the cleanest solution to avoid the exception.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

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Note

Low Risk
Low risk: change is limited to iterating over snapshots of requests to prevent concurrent modification during timeout checks/recovery state updates, with minimal behavioral impact beyond slightly extra allocations.

Overview
Prevents a ConcurrentModificationException in SidecarRetriever.checkPendingRequests() by iterating over snapshots of the requests map (values and entry set) so callbacks can remove entries safely during iteration.

Adds a changelog bugfix entry noting the concurrency fix.

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