Use WeakReference to avoid memory leaks in ArrayPool ThreadStatic field

[konard]

Summary

• Replace the ThreadStatic field _threadInstance with a WeakReference to prevent memory leaks
• Update the ThreadInstance property to properly handle the WeakReference pattern
• Maintain all existing functionality while allowing garbage collection of unused ArrayPool instances

Details

This change addresses issue #99 by replacing the strong reference ThreadStatic field with a WeakReference. The original implementation kept ArrayPool instances alive for the entire thread lifetime, which could lead to memory leaks in long-running applications.

Changes Made

• Changed _threadInstance field from ArrayPool<T> to WeakReference<ArrayPool<T>>
• Updated ThreadInstance property to use TryGetTarget() method for safe weak reference access
• Added fallback logic to create a new ArrayPool instance when the weak reference is collected

Benefits

• Memory leak prevention: ArrayPool instances can now be garbage collected when memory pressure is high
• Backward compatibility: All existing APIs continue to work without changes
• Thread safety: ThreadStatic behavior is preserved
• Performance: Minimal overhead added for memory management benefits

Testing

• All existing unit tests pass (20/20)
• Build completes successfully
• No breaking changes to the public API

Fixes #99

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