🧪 Add tests for _file_size_mb

[marko1olo]

🎯 What: Added missing tests for the _file_size_mb function in main.py.
📊 Coverage: Covered successful file size calculations for small and large sizes, and handled OSError when a file is not found, using mocks to simulate file sizes and errors.
✨ Result: Improved test coverage and reliability for file size utility function.

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PR created automatically by Jules for task 17412378851342186062 started by @marko1olo

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