fix(smolvla2): eagerly import robot submodules before get_choice_class

``RobotConfig._choice_registry`` is populated as a side-effect of
each robot's ``@RobotConfig.register_subclass`` decorator running,
and those decorators only fire when the corresponding
``lerobot.robots.<name>`` module is imported. The package's
``__init__.py`` doesn't import them — instead ``make_robot_from_config``
does it lazily in its big if/elif chain.

``_build_robot`` jumped the gun: called ``RobotConfig.get_choice_class
(robot_type)`` before any robot module had been imported, so the
registry was empty and every ``--robot.type=<X>`` produced
``KeyError: 'X'`` (e.g. ``KeyError: 'omx_follower'``).

Walk ``lerobot.robots``'s submodules via ``pkgutil.iter_modules`` and
``importlib.import_module`` each one before the lookup. ~200ms on the
first invocation, negligible for an autonomous run. On a real
``KeyError`` (typo / unsupported robot), raise a clean ``ValueError``
listing the registry's available choices instead of a bare KeyError.

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

src/lerobot/scripts/lerobot_smolvla2_runtime.py

@@ -452,14 +452,41 @@ def _build_robot(
     commanded joint position relative to the current measured one
     before issuing it on the bus.
     """
+    import importlib  # noqa: PLC0415
     import json  # noqa: PLC0415
+    import pkgutil  # noqa: PLC0415
 
+    import lerobot.robots as _robots_pkg  # noqa: PLC0415
     from lerobot.robots import (  # noqa: PLC0415
         RobotConfig,
         make_robot_from_config,
     )
 
+    # ``RobotConfig._choice_registry`` is populated lazily — each robot's
+    # ``config_<name>.py`` calls ``@RobotConfig.register_subclass`` at
+    # import time. ``lerobot.robots/__init__.py`` doesn't import the
+    # individual robot packages, so ``get_choice_class(robot_type)``
+    # raises ``KeyError`` until at least one robot module has been
+    # imported. Mirror what ``make_robot_from_config`` does internally:
+    # walk the robots package's submodules and import each so the
+    # decorator side-effect runs. Slow only on the first call (~200ms
+    # for ~10 dataclass modules); negligible for an autonomous run that
+    # then loops at ctrl_hz for minutes.
+    for _modinfo in pkgutil.iter_modules(_robots_pkg.__path__):
+        if _modinfo.name.startswith("_"):
+            continue
+        try:
+            importlib.import_module(f"lerobot.robots.{_modinfo.name}")
+        except Exception as exc:  # noqa: BLE001
+            logger.debug("could not import lerobot.robots.%s: %s", _modinfo.name, exc)
+
+    try:
         cls = RobotConfig.get_choice_class(robot_type)
+    except KeyError as exc:
+        available = sorted(RobotConfig._choice_registry.keys())
+        raise ValueError(
+            f"Unknown robot type {robot_type!r}. Available choices: {available}"
+        ) from exc
     kwargs: dict[str, Any] = {}
     if robot_port:
         kwargs["port"] = robot_port