FIX test_video_scorer: use tmp_path for sample video so tests are xdist-safe

[immu4989]

Fixes #1794.

The video_converter_sample_video fixture in tests/unit/score/test_video_scorer.py wrote its sample MP4 to a CWD-relative path ("test_video.mp4"). Under pytest-xdist -n auto (default --dist=load), the 17 tests in this file shard across workers that share the same CWD, racing on a single file and producing flaky FileNotFoundErrors.

CI runs pytest -n 4 --dist=loadfile (Makefile) which pins each file to one worker and masks the race, so this only bites devs running the xdist default locally.

Fix

Inject tmp_path into the fixture and place the sample video there. Per-test temp dirs are unique across workers, so the race vanishes. The same pattern is already used by tests/unit/prompt_converter/test_add_image_video_converter.py; this brings test_video_scorer.py in line.

The explicit os.path.exists / os.remove teardown is removed -- tmp_path cleans itself up.

Verification

# Pre-fix:
pytest tests/unit/score/test_video_scorer.py -n auto   -> 2 failed, 3 passed
pytest tests/unit -n auto                              -> 2 failed in 99s

# Post-fix:
pytest tests/unit/score/test_video_scorer.py -n auto   -> 17 passed in 4.12s
pytest tests/unit -n auto                              -> 8215 passed in 37s

[romanlutz]

Great idea!

[immu4989]

Appreciate the feedback. Thank you !