🧪 Add test for is_rknn_compatible

[ManupaKDU]

🎯 What: The testing gap addressed
The is_rknn_compatible method in frigate/util/rknn_converter.py was not tested. It has clear behavior based on string matching and checking support arrays, so it was a prime candidate for tests to ensure future changes do not introduce bugs.

📊 Coverage: What scenarios are now tested
The new test module frigate/test/test_rknn_converter.py tests:

• When the SoC type cannot be determined.
• When an unsupported SoC is present.
• When a supported SoC is present along with valid/invalid explicit model_type values.
• When a supported SoC is present, model_type is missing, but a valid/invalid model type is inferred.

✨ Result: The improvement in test coverage
The test coverage for frigate/util/rknn_converter.py has been significantly improved, directly testing the logical branches of is_rknn_compatible. Test fixtures make use of unittest.mock to ensure isolation and robustness when dependencies are not present.

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PR created automatically by Jules for task 921988146595792646 started by @manupawickramasinghe

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