fix: BROS-494: Fix <Chat> predictions validation logic (#646)

Co-authored-by: nik <nik@heartex.net>

Author: niklub

src/label_studio_sdk/label_interface/control_tags.py

@@ -986,7 +986,8 @@ def to_json_schema(self):
                         "role": {"type": "string"},
                         "content": {"type": "string"},
                         "createdAt": {"type": "number"}
-                    }
+                    },
+                    "additionalProperties": True
                 }
             },
             "description": f"Chat message for {self.to_name[0]}"