🧪 [testing improvement] Add tests for Gen5ComputeSystemRegisters.TryGetExpression

[manupawickramasinghe]

🎯 What: Added tests for the TryGetExpression method within the shader IR components to verify register mapping to GLSL expressions.
📊 Coverage: Covered all known register mappings (WorkGroup X/Y/Z, ThreadGroupSize), unmatched register fallback, and null properties edge cases.
✨ Result: Improved test coverage and verification for shader evaluation state and compute system registers.

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PR created automatically by Jules for task 3821128137305832918 started by @manupawickramasinghe

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