fix(smolvla2): use TransitionKey enum (not .value) as transition keys

``EnvTransition`` is declared as a ``TypedDict`` keyed by
``TransitionKey.OBSERVATION.value`` (the string ``'observation'``),
but every concrete ``ProcessorStep`` in the pipeline indexes the
transition with the enum *member* (``transition[TransitionKey.
OBSERVATION]`` / ``transition.get(TransitionKey.OBSERVATION)``).
Those are two different keys in a Python dict — string key vs enum
key — so steps couldn't find the observation we'd placed under the
string variant, and bailed every tick with
``ObservationProcessorStep requires an observation in the
transition``.

Build the transition with the enum members directly. Matches how
``BatchProcessor``, ``RelativeActionProcessor``, ``HilProcessor``,
etc. read the dict.

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

src/lerobot/scripts/lerobot_smolvla2_runtime.py

@@ -629,21 +629,28 @@ def _build_robot_observation_provider(
             return None
 
         if preprocessor is not None:
-            transition: dict[str, Any] = {
+            # ``EnvTransition``'s TypedDict is declared with
-                TransitionKey.OBSERVATION.value: obs_tensors,
+            # ``TransitionKey.OBSERVATION.value`` as keys, but every
-                TransitionKey.ACTION.value: None,
+            # ProcessorStep in the pipeline does
-                TransitionKey.REWARD.value: None,
+            # ``transition.get(TransitionKey.OBSERVATION)`` / indexes
-                TransitionKey.DONE.value: None,
+            # with the *enum member* — not the string ``.value``. Build
-                TransitionKey.TRUNCATED.value: None,
+            # the dict with enum keys so the steps actually find the
-                TransitionKey.INFO.value: None,
+            # observation.
-                TransitionKey.COMPLEMENTARY_DATA.value: {},
+            transition: dict[Any, Any] = {
+                TransitionKey.OBSERVATION: obs_tensors,
+                TransitionKey.ACTION: None,
+                TransitionKey.REWARD: None,
+                TransitionKey.DONE: None,
+                TransitionKey.TRUNCATED: None,
+                TransitionKey.INFO: None,
+                TransitionKey.COMPLEMENTARY_DATA: {},
             }
             try:
                 transition = preprocessor(transition)
             except Exception as exc:  # noqa: BLE001
                 logger.warning("preprocessor failed on robot observation: %s", exc)
                 return None
-            obs_tensors = transition.get(TransitionKey.OBSERVATION.value) or {}
+            obs_tensors = transition.get(TransitionKey.OBSERVATION) or {}
 
         observation = {
             k: v