🧪 [testing improvement] Add unit tests for Grid<T> collection

[johnstrand]

🎯 What: The testing gap addressed\nMissing unit tests for Grid<T> collection in GameUtils.Types.Collections.\n\n📊 Coverage: What scenarios are now tested\n- Constructors (valid dimensions, invalid/negative dimensions throwing ArgumentOutOfRangeException, array initialization, and data length mismatch throwing ArgumentException).\n- Indexers using both integer (x, y) coordinates and Vector2 coordinates, including out-of-bounds access throwing IndexOutOfRangeException.\n- TryGet and TrySet safe access methods for integer and Vector2 coordinates.\n- IsInBounds validation for integer and Vector2 coordinates.\n- Clear resetting all elements to default values.\n- Fill overloads (constant value, Func<int, int, T> factory, Func<Vector2, T> factory).\n- IEnumerable<T> and non-generic IEnumerable iteration.\n\n✨ Result: The improvement in test coverage\n100% method and branch coverage for Grid<T>, adding 22 new unit tests to the codebase without introducing regressions.

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PR created automatically by Jules for task 9923669839520695550 started by @johnstrand

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