fix(ag-ui): list typing under python 3.9

Python 3.9 doesn't support multiple types in a single list type hint so
switch to Union for compatibility.

Author: stevenh

pydantic_ai_slim/pydantic_ai/ag_ui.py

@@ -21,6 +21,7 @@
     NoReturn,
     Protocol,
     TypeVar,
+    Union,
     cast,
     runtime_checkable,
 )
@@ -1016,7 +1017,7 @@ class _RequestStreamContext:
 
     message_id: str = ''
     last_tool_call_id: str | None = None
-    part_ends: list[BaseEvent | None] = field(default_factory=lambda: list[BaseEvent | None]())
+    part_ends: list[BaseEvent | None] = field(default_factory=lambda: list[Union[BaseEvent, None]]())
     local_tool_calls: set[str] = field(default_factory=set)
 
     def new_message_id(self) -> str: