chore: replace hard-coded obs values with constants throughout all the source code (#2037)

* chore: replace hard-coded OBS values with constants throughout all the source code

* chore(tests): replace hard-coded OBS values with constants throughout all the test code

tests/async_inference/test_policy_server.py

@@ -23,6 +23,7 @@ import pytest
 import torch
 
 from lerobot.configs.types import PolicyFeature
+from lerobot.utils.constants import OBS_STATE
 from tests.utils import require_package
 
 # -----------------------------------------------------------------------------
@@ -44,7 +45,7 @@ class MockPolicy:
 
     def predict_action_chunk(self, observation: dict[str, torch.Tensor]) -> torch.Tensor:
         """Return a chunk of 20 dummy actions."""
-        batch_size = len(observation["observation.state"])
+        batch_size = len(observation[OBS_STATE])
         return torch.zeros(batch_size, 20, 6)
 
     def __init__(self):
@@ -77,7 +78,7 @@ def policy_server():
 
     # Add mock lerobot_features that the observation similarity functions need
     server.lerobot_features = {
-        "observation.state": {
+        OBS_STATE: {
             "dtype": "float32",
             "shape": [6],
             "names": ["joint1", "joint2", "joint3", "joint4", "joint5", "joint6"],