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

MANIFEST.in

@@ -1,2 +1,3 @@
 include src/lerobot/templates/lerobot_modelcard_template.md
 include src/lerobot/datasets/card_template.md
+include src/lerobot/envs/metaworld_config.json