Fix metaworld_config.json not bundled in pip installs and AttributeError crash (#3017)

1. Include metaworld_config.json in package distributions by adding it to
   both MANIFEST.in (for sdist) and pyproject.toml package-data (for wheels).
   Without this, pip-installed lerobot raises FileNotFoundError when
   importing the metaworld environment.

2. Fix crash in sanity_check_dataset_name where the error message accesses
   policy_cfg.type when policy_cfg is None, raising AttributeError instead
   of the intended ValueError.

Fixes #2958

pyproject.toml

@@ -214,6 +214,9 @@ lerobot-edit-dataset="lerobot.scripts.lerobot_edit_dataset:main"
 lerobot-setup-can="lerobot.scripts.lerobot_setup_can:main"
 
 # ---------------- Tool Configurations ----------------
+[tool.setuptools.package-data]
+lerobot = ["envs/*.json"]
+
 [tool.setuptools.packages.find]
 where = ["src"]