fix: prevent crash when jweak points to null

[edusperoni]

Occasionally (mostly on android API 26.1+) a jweak reference might point to NULL even when the actual Java object exists.

I believe this is a JNI/Android bug, but as a workaround, we now check if this reference is null, and if so we evict the key from the cache which then is going to re-fetch it and error out if it's actually null in Java

[Copilot]

Pull Request Overview

This PR adds cache validation functionality to the LRUCache implementation to check for stale weak global references in the ObjectManager. It introduces a callback mechanism to validate cached entries before they are used, preventing the use of invalidated Java weak references.

Key changes:

• Added a validation callback parameter to LRUCache constructor to check if cached entries are still valid
• Implemented validation logic that evicts invalid cache entries before returning them
• Switched from std::unordered_map to robin_hood::unordered_map for improved performance

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
test-app/runtime/src/main/cpp/ObjectManager.h	Added declaration for ValidateWeakGlobalRefCallback method
test-app/runtime/src/main/cpp/ObjectManager.cpp	Implemented validation callback and updated cache initialization to use it; removed unused variable
test-app/runtime/src/main/cpp/LRUCache.h	Added validation callback support, implemented evictKey helper method, and switched to robin_hood hash map
test-app/runtime/src/main/cpp/JEnv.h	Added isSameObject helper method for JNI object comparison

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