psycopg_pool.PoolTimeout: couldn‘t get a connection after 120.00 sec异常

langgraph使用AsyncConnectionPool方式连接postgres一段时间（一小时）后失效了，报异常psycopg_pool.PoolTimeout

参考官方样例：https://langchain-ai.github.io/langgraph/how-tos/persistence/#use-in-production

解决办法：使用AsyncPostgresSaver.from_conn_string替代AsyncPostgresSaver(AsyncConnectionPool)连接postgres