Improve FFmpeg Error Handling in Video Stream Tests

[rquidute]

What changed

This PR implements comprehensive FFmpeg validation and error handling:

1. Pre-validation Check - FFmpeg is checked before WebSocket connection, preventing the stream from being marked as "ready" when FFmpeg is missing
2. Clear User Feedback - Displays formatted installation instructions when FFmpeg is not detected
3. Graceful Abort - Sends a CANCELLED response to the backend to properly abort test execution instead of hanging
4. Exception Handling - Catches and handles all FFmpeg-related errors in the background async task

Also some minors formatting and import sort fixes was performed in this PR.

Related Issue

project-chip/certification-tool#844

Testing

Unit test passing

Message displayed to the user when no ffmpeg is found

[gemini-code-assist]

Summary of Changes

Hello @rquidute, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the video stream tests by implementing robust FFmpeg validation and error handling. These changes ensure that the test execution is aborted gracefully and the user is provided with actionable feedback when FFmpeg is not properly configured.

Highlights

• FFmpeg Validation: Implements a pre-validation check for FFmpeg before establishing a WebSocket connection, preventing the stream from being marked as 'ready' when FFmpeg is missing.
• User Feedback: Provides clear, formatted installation instructions to the user when FFmpeg is not detected.
• Graceful Abort: Sends a CANCELLED response to the backend to properly abort test execution instead of hanging when video stream initialization fails.
• Exception Handling: Implements comprehensive exception handling for FFmpeg-related errors within the background async task.

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[gemini-code-assist]

Code Review

This pull request introduces comprehensive error handling for FFmpeg in the video stream tests, which is a great improvement for robustness and user experience. The changes include pre-validating the FFmpeg installation, providing clear user feedback on missing dependencies, and gracefully aborting tests on failure. The implementation is solid and directly addresses the issue. I've added a couple of suggestions to further enhance maintainability by refactoring duplicated code and improving the robustness of exception handling.