Fix datetime serialization in _dto_to_dict

Use dto_to_dict() from shared/contracts/dtos.py for dataclass
serialization instead of raw dataclasses.asdict(). This ensures
datetimes are converted to ISO format strings (not RFC 2822 from
jsonify), matching what dto_from_dict() expects on the receiving end.

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

shared/sx/primitives_io.py

@@ -226,8 +226,8 @@ def _dto_to_dict(obj: Any) -> dict[str, Any]:
     without parsing date strings.
     """
     if hasattr(obj, "__dataclass_fields__"):
-        import dataclasses
-        d = dataclasses.asdict(obj)
+        from shared.contracts.dtos import dto_to_dict
+        return dto_to_dict(obj)
     elif hasattr(obj, "_asdict"):
         d = dict(obj._asdict())
     elif hasattr(obj, "__dict__"):