fix lint

Author: DouweM

pydantic_ai_slim/pydantic_ai/ui/messages_builder.py

@@ -11,15 +11,17 @@ class MessagesBuilder:
 
     messages: list[ModelMessage] = field(default_factory=list)
 
-    def add(self, part: ModelRequest | ModelResponse | ModelRequestPart | ModelResponsePart) -> None:
+    def add(self, part: ModelRequestPart | ModelResponsePart) -> None:
         """Add a new part, creating a new request or response message if necessary."""
         last_message = self.messages[-1] if self.messages else None
         if isinstance(part, get_union_args(ModelRequestPart)):
+            part = cast(ModelRequestPart, part)
             if isinstance(last_message, ModelRequest):
-                last_message.parts = [*last_message.parts, cast(ModelRequestPart, part)]
+                last_message.parts = [*last_message.parts, part]
             else:
                 self.messages.append(ModelRequest(parts=[part]))
         else:
+            part = cast(ModelResponsePart, part)
             if isinstance(last_message, ModelResponse):
                 last_message.parts = [*last_message.parts, part]
             else: