refactor(datasets): replace untyped dict with typed DatasetInfo dataclass

Introduce typed DatasetInfo dataclass to replace untyped dict representation of info.json.

Changes:
- Add DatasetInfo dataclass with explicit fields and validation
- Implement __post_init__ for shape conversion (list ↔ tuple)
- Add dict-style compatibility layer (__getitem__, __setitem__, .get())
- Add from_dict() and to_dict() for JSON serialization
- Update io_utils to use load_info/write_info with DatasetInfo
- Update dataset utilities and metadata to use attribute access
- Remove aggregate.py dict-style field access
- Add tests fixture support for DatasetInfo

Benefits:
- Type safety with IDE auto-completion
- Validation at construction time
- Explicit schema documentation

tests/datasets/test_dataset_metadata.py

@@ -161,7 +161,7 @@ def test_init_loads_existing_metadata(tmp_path, lerobot_dataset_metadata_factory
 
     assert meta.total_episodes == 3
     assert meta.total_frames == 150
-    assert meta.fps == info["fps"]
+    assert meta.fps == info.fps
 
 
 # ── Property accessors ───────────────────────────────────────────────