feat(cache): correct LruCache locking + add tests

[zamderax]

Fix LRU cache concurrency bug and add tests

What changed

• Fixed potential deadlock in LruCache by ensuring callers hold locks instead of internal methods attempting to re-acquire them
• Replaced O(n) list searches with O(1) dictionary lookups for better performance
• Added guard to prevent storing nil values
• Added comprehensive unit tests (Swift Testing) covering set/get, eviction order, and cache updates
• Added documentation for public methods

Why

• Previous implementation had nested locking in both public and internal methods, which could cause deadlocks under re-entrance
• Improves both thread safety and performance

Testing

• All tests pass locally (macOS 15, Swift 6.1.2)
• New tests verify cache behavior including eviction order and capacity limits

Follow-ups

• Consider exposing cache capacity via API for better observability (currently kept internal)

[hsluoyz]

🎉 This PR is included in version 1.10.0 🎉

The release is available on GitHub release

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