fix: integrate PR #3396 review feedback

- processor(vlabench): drop the no-op `VLABenchProcessorStep` and
  let `VLABenchEnv` fall back to the identity `get_env_processors`
  default. Removes the class, its export, and its override.
- envs(vlabench): hoist `import cv2` to module scope; propagate the
  gym `seed` into the inner dm_control env (re-seeding `task.random`
  and the env's `_random_state`) so layout sampling is reproducible.
- docs(vlabench): mirror the CI eval command across all four
  snippets (`--eval.use_async_envs=false`, `--policy.device=cuda`,
  and the `rename_map` for image/second_image/wrist_image →
  camera1/2/3). Intro now calls out that LeRobot exposes 43 of
  VLABench's 100 upstream task categories via `--env.task`.
- docker(vlabench): drop the `|| echo WARN` fallback on the gdown
  asset-download path so a genuine download failure surfaces
  instead of being masked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker/Dockerfile.benchmark.vlabench

@@ -68,9 +68,8 @@ p = snapshot_download(repo_id='${VLABENCH_ASSETS_REPO}', repo_type='dataset', \
     local_dir='${ASSETS_DIR}', allow_patterns=['obj/**', 'scenes/**']); \
 print('snapshot_download returned:', p)"; \
     else \
-        echo "No VLABENCH_ASSETS_REPO set — falling back to gdown (best-effort)" && \
-        python ~/VLABench/scripts/download_assets.py --choice all || \
-          echo "WARN: VLABench asset download failed (likely Google Drive quota)."; \
+        echo "No VLABENCH_ASSETS_REPO set — falling back to gdown" && \
+        python ~/VLABench/scripts/download_assets.py --choice all; \
     fi && \
     python -c "\
 from pathlib import Path; \