Add dataset_infos.json with explicit string features

- Fixes 'Feature type Json not found' error in dataset viewer
- Explicitly declares all fields as Value(string) to prevent auto-inference issues
- Required for nested/dict fields (e.g. v-Fable output field)

dataset_infos.json

@@ -0,0 +1,12 @@
+{
+  "default": {
+    "features": {
+      "instruction": {"dtype": "string", "_type": "Value"},
+      "input": {"dtype": "string", "_type": "Value"},
+      "output": {"dtype": "string", "_type": "Value"}
+    },
+    "splits": {
+      "train": {"name": "train", "num_bytes": 440000000, "num_examples": 50000}
+    }
+  }
+}