Fix _dto_to_dict for slots=True dataclasses

The defquery conversion routes inter-service results through
_dto_to_dict which checked __dict__ (absent on slots dataclasses),
producing {"value": obj} instead of proper field dicts. This broke
TicketDTO deserialization in the cart app. Check __dataclass_fields__
first and use dataclasses.asdict() for correct serialization.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/primitives_io.py

@@ -225,7 +225,10 @@ def _dto_to_dict(obj: Any) -> dict[str, Any]:
     keys for any datetime-valued field so sx handlers can build URL paths
     without parsing date strings.
     """
-    if hasattr(obj, "_asdict"):
+    if hasattr(obj, "__dataclass_fields__"):
+        import dataclasses
+        d = dataclasses.asdict(obj)
+    elif hasattr(obj, "_asdict"):
         d = dict(obj._asdict())
     elif hasattr(obj, "__dict__"):
         d = {k: v for k, v in obj.__dict__.items() if not k.startswith("_")}