The pyav fallback routed through lerobot's decode_video_frames(backend=
"pyav"), which uses torchvision.io.VideoReader — removed in torchvision
0.23+. On modern torch stacks (e.g. vllm-openai with torchvision 0.26)
both torchcodec and that path fail, leaving interjection/vqa prompts
without visual context.
Add _decode_frames_av: a self-contained PyAV decoder that picks the
nearest frame per timestamp. It is the always-available tail of the
decoder chain (torchcodec -> pyav) and the target of --video_backend=pyav.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>