Fix type annotations for Agent.iter() (#2168)

Author: erosennin

pydantic_ai_slim/pydantic_ai/agent.py

@@ -500,7 +500,7 @@ async def main():
     @overload
     def iter(
         self,
-        user_prompt: str | Sequence[_messages.UserContent] | None,
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         output_type: None = None,
         message_history: list[_messages.ModelMessage] | None = None,
@@ -516,7 +516,7 @@ def iter(
     @overload
     def iter(
         self,
-        user_prompt: str | Sequence[_messages.UserContent] | None,
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         output_type: OutputSpec[RunOutputDataT],
         message_history: list[_messages.ModelMessage] | None = None,
@@ -533,7 +533,7 @@ def iter(
     @deprecated('`result_type` is deprecated, use `output_type` instead.')
     def iter(
         self,
-        user_prompt: str | Sequence[_messages.UserContent] | None,
+        user_prompt: str | Sequence[_messages.UserContent] | None = None,
         *,
         result_type: type[RunOutputDataT],
         message_history: list[_messages.ModelMessage] | None = None,