updated client code to match refactored server code

memgpt/client/client.py

@@ -67,7 +67,7 @@ class Client(object):
 
     def agent_exists(self, agent_id: str) -> bool:
         existing = self.list_agents()
-        return agent_id in existing["agent_names"]
+        return agent_id in [agent["name"] for agent in existing["agents"]]
 
     def create_agent(
         self,