LangGraph - Undefined table 'checkpoints' when using Postgres.

## The fix

[alberto-agudo (commenter)]: You skipped the one-time schema setup. The first time you use PostgresSaver against a database, call checkpointer.setup() before anything else: with PostgresSaver.from_conn_string(DB_URI) as checkpointer: checkpointer.setup(). It creates the checkpoints tables; without it every read/write fails with "Undefined table checkpoints". You only need it once per database, so gate it behind a first-run flag.

Thread: gh:langchain-ai/langgraph#2062