allow updating the same agent that is running to apply changes in agent (#3814)

Author: longcw

livekit-agents/livekit/agents/voice/agent_session.py

@@ -928,7 +928,11 @@ async def _update_activity(
             self._agent = agent
 
             if new_activity == "start":
-                if agent._activity is not None:
+                previous_agent = self._activity.agent if self._activity else None
+                if agent._activity is not None and (
+                    # allow updating the same agent that is running
+                    agent is not previous_agent or previous_activity != "close"
+                ):
                     raise RuntimeError("cannot start agent: an activity is already running")
 
                 self._next_activity = AgentActivity(agent, self)