🧪 Add tests for MetadataEngine SanitizeForPath edge cases

[gonzoga]

🎯 What: The testing gap addressed
This PR addresses the missing unit tests for the private SanitizeForPath method in MetadataEngine.cs by indirectly testing it via the GetSubFolder method using temporary mock MP3 files generated with TagLib.

📊 Coverage: What scenarios are now tested
The new tests cover:

• Artist names containing valid path characters mixed with invalid ones (e.g., AC/DC, The:Who, Artist?Name).
• Edge cases where artist names are empty, whitespace only, or consist entirely of invalid characters (e.g., *:*, ///), correctly falling back to "Unknown" or "Unknown Artist".

✨ Result: The improvement in test coverage
The codebase now safely and robustly verifies how the organizer handles erratic metadata when classifying audio, preventing crashes or filesystem errors upon deployment. To achieve this cleanly in testing, a minimal valid MP3 frame was constructed in memory to ensure TagLib does not throw a CorruptFileException.

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PR created automatically by Jules for task 13391177433328017201 started by @gonzoga

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