fix(agent.py): update handlers type annotation to support both Handle…

…r and AsyncHandler

src/controlflow/agents/agent.py

@@ -238,7 +238,7 @@ async def run_async(
         objective: str,
         *,
         turn_strategy: Optional["TurnStrategy"] = None,
-        handlers: Optional[list["Handler"]] = None,
+        handlers: Optional[list[Union["Handler", "AsyncHandler"]]] = None,
         **task_kwargs,
     ):
         return await controlflow.run_async(