Add reactor cache cleanup

[harshang03]

What does this PR do?
Adds periodic cleanup for reactor client caches and a manual reactor.cleanup runner to release retained objects in long-running reactor processes. Also allows configuring cleanup interval in the reactor engine.

What issues does this PR fix or reference?
Fixes #68660

Previous Behavior
Reactor client cache could retain objects for long periods without explicit cleanup, contributing to memory growth under sustained event load.

New Behavior
Reactor periodically triggers cache cleanup and exposes a runner to request cleanup on demand.

Merge requirements satisfied?
[ ] Docs
[ ] Changelog
[ ] Tests written/updated

Commits signed with GPG?
No

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