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/datasets/test_language_render.py

@@ -2,8 +2,16 @@
 
 import pytest
 
-from lerobot.configs.recipe import MessageTurn, TrainingRecipe
-from lerobot.datasets.language_render import active_at, emitted_at, nth_next, nth_prev, render_sample
+pytest.importorskip("datasets", reason="datasets is required (install lerobot[dataset])")
+
+from lerobot.configs.recipe import MessageTurn, TrainingRecipe  # noqa: E402
+from lerobot.datasets.language_render import (  # noqa: E402
+    active_at,
+    emitted_at,
+    nth_next,
+    nth_prev,
+    render_sample,
+)
 
 
 def persistent_row(role, content, style, timestamp, tool_calls=None, camera=None):