[langgraph] Unable to pass serde to PostgresSaver via from_conn_string

## Observed symptom Unable to pass serde to PostgresSaver in langgraph.checkpoint.postgres via function from_conn_string ### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangGraph rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangGraph (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Related Issues / PRs When using AyncPostgresSaver fromlanggraph.checkpoint.postgres.aio, users can pass serde t ## Literal error text ``` Unable to pass serde to PostgresSaver ``` ## Environment GitHub issue langchain-ai/langgraph #9118 ## Trigger / context ### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangGraph rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangGraph (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Related Issues / PRs When using AyncPostgresSaver fromlanggraph.checkpoint.postgres.aio, users can pass serde to from_conn_string function but unable to pass serde to PostgresSaver in langgraph.checkpoint.postgres via function from_conn_string. The fix is simple keep an argument in from_conn_string function 'serde' and pass it to cls instance as its being returned. ### Reproduction Steps / Example Code (Python) ```python from langgraph.checkpoint.postgres import PostgresSaver conn_str = 'example_conn_str' serde = {} with PostgresSaver.from_conn_string(conn_string=conn_string, serde=serde) as checkpoi ## Source evidence - https://github.com/langchain-ai/langgraph/issues/9118 (2026-10-01T09:53:05Z) — #9118 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: C6f0d95416e ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.