Document ffmpeg 5.1+ for -fps_mode and fail the suite once when older

[HarshRajSinghania]

Fixes #491.

Summary

The test suite uses -fps_mode (src/hflow/video.py, src/hflow/episode.py). That option is not present in FFmpeg 4.4.x (Ubuntu 22.04 default), so a contributor on the distro binary gets encode-path assertion failures that look like their branch broke something.

Motivation

#491 asked for two things: name the minimum in CONTRIBUTING, and replace those three obscure failures with one message that states the required and found versions.

Implementation

• CONTRIBUTING now requires ffmpeg 5.1 or newer and names -fps_mode so the floor can be re-derived.
• A session-scoped autouse fixture in tests/conftest.py probes ffmpeg -version once. If the parsed version is older than 5.1 it calls pytest.exit with required vs found. Missing ffmpeg or an unparseable banner is left alone so existing skip/fail paths still apply.
• No check was added to the encode path.

How the 5.1 floor was established: -fps_mode replaced -vsync in the FFmpeg 5.1 CLI. This environment's ffmpeg is 6.1.1-3ubuntu5, which lists -fps_mode in ffmpeg -h full. Ubuntu 22.04's 4.4.2 does not.

Testing

• PYTHONPATH=. pytest -q tests/test_ffmpeg_version_guard.py — 2 passed
• Parser checks for Ubuntu 4.4.2, n5.1.2, and 6.1.1 banners
• Full uv sync --locked --all-extras / ruff / ty / full pytest were not run here (no project venv). Local ffmpeg is 6.1.1, which is above the floor.

[github-actions]

👋 Hi @HarshRajSinghania — thank you so much for your first contribution to HFlow!

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[kstonekuan]

ruff format --check fails on tests/conftest.py:51, so this would turn main red.

pytest.exit ends the whole session. Someone working on curation with an old ffmpeg now cannot run any test. #491 asked for the ffmpeg-dependent tests to skip or fail, not the run to abort.

Rest looks right, and naming -fps_mode in CONTRIBUTING so the floor can be re-derived is the part that keeps it true.