Fix loop graceful shutdown (#4394)

openhands/server/session/agent_session.py

@@ -140,7 +140,7 @@ class AgentSession:
             await self.security_analyzer.close()
 
         if self.loop:
-            self.loop.call_soon_threadsafe(self.loop.stop)
+            self.loop.stop()
 
         self._closed = True