get_safe_default_video_backend picks torchcodec when installed but unloadable (missing FFmpeg shared libraries)

[ahmedsohail2003]

Description

get_safe_default_video_backend() promotes torchcodec to the default video backend whenever importlib.util.find_spec("torchcodec") succeeds. But installed is not the same as loadable: torchcodec links against FFmpeg shared libraries at runtime, which are commonly absent on Windows (and on minimal Linux images). find_spec still sees the package, the default becomes torchcodec, and every dataset video decode then crashes with a DLL-load RuntimeError unless the user knows to pass video_backend="pyav" explicitly.

Reproduction

1. Windows machine without FFmpeg shared libraries on PATH
2. pip install "lerobot[dataset]" (installs the torchcodec wheel fine)
3. Load any video dataset with defaults: LeRobotDataset("lerobot/pusht")[0]
4. Crash at first decode; workaround is video_backend="pyav"

Expected behavior

The safe default should verify the backend actually loads (e.g., probe torchcodec.decoders, which decoding imports anyway) and fall back to pyav with an actionable warning. PR attached.