fix(annotate): decode keyframes via PyAV directly

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>

tests/annotations/test_frames.py

@@ -29,12 +29,18 @@ LeRobot dataset on disk.
 
 from __future__ import annotations
 
+import shutil
+import subprocess
+from pathlib import Path
+
 import pytest
+import torch
 
 pytest.importorskip("datasets", reason="datasets is required (install lerobot[dataset])")
 
 from lerobot.annotations.steerable_pipeline.frames import (  # noqa: E402
     VideoFrameProvider,
+    _decode_frames_av,
 )
 
 
@@ -64,3 +70,53 @@ def test_videoframeprovider_has_a_lock_for_concurrent_use():
     )
     assert lock_field is not None
     assert lock_field.default_factory is threading.Lock
+
+
+@pytest.fixture
+def sample_video(tmp_path: Path) -> Path:
+    """A 3 s 10 fps test-pattern mp4, written with ffmpeg."""
+    if shutil.which("ffmpeg") is None:
+        pytest.skip("ffmpeg not available")
+    out = tmp_path / "sample.mp4"
+    subprocess.run(
+        [
+            "ffmpeg", "-y", "-f", "lavfi",
+            "-i", "testsrc=duration=3:size=160x120:rate=10",
+            "-pix_fmt", "yuv420p", str(out),
+        ],
+        check=True,
+        capture_output=True,
+    )
+    return out
+
+
+def test_decode_frames_av_returns_one_uint8_frame_per_timestamp(sample_video: Path) -> None:
+    """``_decode_frames_av`` decodes via PyAV directly — no torchcodec/torchvision.
+
+    This is the always-available fallback: torchcodec is unusable in some
+    containers and lerobot's ``pyav`` backend routes through the removed
+    ``torchvision.io.VideoReader``.
+    """
+    timestamps = [0.0, 1.0, 2.5]
+    frames = _decode_frames_av(sample_video, timestamps)
+
+    assert len(frames) == len(timestamps)
+    for frame in frames:
+        assert isinstance(frame, torch.Tensor)
+        assert frame.dtype == torch.uint8
+        assert frame.shape == (3, 120, 160)
+
+
+def test_decode_frames_av_picks_nearest_frame(sample_video: Path) -> None:
+    """Repeated and out-of-order timestamps each resolve to the nearest frame."""
+    frames = _decode_frames_av(sample_video, [2.0, 0.0, 2.0])
+
+    assert len(frames) == 3
+    assert torch.equal(frames[0], frames[2])
+    assert not torch.equal(frames[0], frames[1])
+
+
+def test_decode_frames_av_raises_on_missing_file(tmp_path: Path) -> None:
+    """A missing video surfaces as an exception the caller can fall back on."""
+    with pytest.raises(Exception):  # noqa: B017, PT011
+        _decode_frames_av(tmp_path / "does_not_exist.mp4", [0.0])