fix(language): keep base install green — drop processor re-export, gate dataset-extra tests

`lerobot.processor` re-exported `RenderMessagesStep` at the package
level, so importing anything from `lerobot.processor` pulled in
`lerobot.datasets.language` → `lerobot.datasets/__init__.py` →
`require_package("datasets")`, which fails in the Tier 1 base install
that intentionally omits the `[dataset]` extra. The chain bricked
collection for unrelated suites (`tests/policies/pi0_pi05/...`,
`tests/envs/...`, etc.).

* Stop re-exporting `RenderMessagesStep` from `lerobot.processor`. The
  only consumer (the test) already imports from the submodule.
  Document the deliberate omission in the module docstring.
* Add `pytest.importorskip("datasets", ...)` (and `pandas` where
  needed) at the top of the four PR-added tests that exercise the
  language stack:
  - tests/datasets/test_language.py
  - tests/datasets/test_language_render.py
  - tests/processor/test_render_messages_processor.py
  - tests/utils/test_collate.py

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

tests/processor/test_render_messages_processor.py

@@ -1,11 +1,15 @@
 #!/usr/bin/env python
 
-import torch
+import pytest
 
-from lerobot.configs.recipe import MessageTurn, TrainingRecipe
-from lerobot.processor.converters import create_transition
-from lerobot.processor.render_messages_processor import RenderMessagesStep
-from lerobot.types import TransitionKey
+pytest.importorskip("datasets", reason="datasets is required (install lerobot[dataset])")
+
+import torch  # noqa: E402
+
+from lerobot.configs.recipe import MessageTurn, TrainingRecipe  # noqa: E402
+from lerobot.processor.converters import create_transition  # noqa: E402
+from lerobot.processor.render_messages_processor import RenderMessagesStep  # noqa: E402
+from lerobot.types import TransitionKey  # noqa: E402
 
 
 def test_render_messages_step_noops_without_language_columns():