🧪 Add edge case tests for get_soc_type in rknn_converter

[ManupaKDU]

🎯 What: Added tests for get_soc_type function inside frigate/util/rknn_converter.py to properly cover the file reading logic and edge cases where the device tree file does not exist.
📊 Coverage: Covered normal execution paths returning None for Nvidia models, proper parsing for Rockchip devices, and the missing edge case for FileNotFoundError.
✨ Result: Improved robustness by testing the error path and providing safety net tests for SoC parsing logic, directly addressing the testing gap requested.

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PR created automatically by Jules for task 4240969424086872374 started by @manupawickramasinghe

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