fix(video): replace assertions with proper exceptions in video frame decoding (#3016)

Replaced assert statements with FrameTimestampError exceptions in
decode_video_frames_torchvision and decode_video_frames_torchcodec.

Assertions are unsuitable for runtime validation because they can be
silently disabled with python -O, and they produce unhelpful
AssertionError tracebacks. The codebase already defines
FrameTimestampError for this exact purpose but it was only used
in one of the three validation sites.

Also removed AssertionError from the except clause in
LeRobotDataset.__init__, which was masking video timestamp errors
by silently triggering a dataset re-download instead of surfacing
the actual problem.

src/lerobot/datasets/lerobot_dataset.py

@@ -747,7 +747,7 @@ class LeRobotDataset(torch.utils.data.Dataset):
             # Check if cached dataset contains all requested episodes
             if not self._check_cached_episodes_sufficient():
                 raise FileNotFoundError("Cached dataset doesn't contain all requested episodes")
-        except (AssertionError, FileNotFoundError, NotADirectoryError):
+        except (FileNotFoundError, NotADirectoryError):
             if is_valid_version(self.revision):
                 self.revision = get_safe_version(self.repo_id, self.revision)
             self.download(download_videos)