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_lerobot_dataset.py

@@ -80,18 +80,18 @@ def _write_dataset_tree(
     )
     tasks = tasks_factory(total_tasks=1)
     episodes = episodes_factory(
-        features=info["features"],
-        fps=info["fps"],
+        features=info.features,
+        fps=info.fps,
         total_episodes=1,
         total_frames=3,
         tasks=tasks,
     )
-    stats = stats_factory(features=info["features"])
+    stats = stats_factory(features=info.features)
     hf_dataset = hf_dataset_factory(
-        features=info["features"],
+        features=info.features,
         tasks=tasks,
         episodes=episodes,
-        fps=info["fps"],
+        fps=info.fps,
     )
 
     create_info(root, info)