🧪 Test ensure_rknn_toolkit behavior

[ManupaKDU]

🎯 What: The issue requested tests for ensure_rknn_toolkit in frigate/util/rknn_converter.py and pointed out that the function needed to be updated to match the intended logic (dynamically installing rknn-toolkit2 if it's not available).
📊 Coverage: The function ensure_rknn_toolkit is now tested for four scenarios:

• rknn is already installed.
• Dynamic installation succeeds via subprocess.check_call.
• Dynamic installation fails because pip fails (subprocess.CalledProcessError).
• Dynamic installation fails because rknn still can't be imported after pip succeeds (ImportError).
  ✨ Result: Test coverage for this functionality is vastly improved. The function itself was also updated to include the dynamic pip installation behavior.

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PR created automatically by Jules for task 3740701426382972987 started by @manupawickramasinghe

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