🧪 Add tests for Mat4 matrix multiplication

[google-labs-jules]

🎯 What: Addressed missing tests for Mat4::operator*.
📊 Coverage: Added test scenarios covering Identity * A, A * Identity, and arbitrary A * B matrix multiplications.
✨ Result: Improved overall test coverage for core matrix math operations, preventing future regressions.

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PR created automatically by Jules for task 6484538341963640986 started by @EricBorges2019

[google-labs-jules]

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