[Feature]: (LangGraph) Pass `durability` to `.astream_values`

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

### What

Pass [durability](https://docs.langchain.com/oss/python/langgraph/durable-execution#durability-modes) to `astream_events` [here](https://github.com/ag-ui-protocol/ag-ui/blob/main/integrations/langgraph/python/ag_ui_langgraph/agent.py#L552):
```python
kwargs["durability"] = "" # add somehow
stream = self.graph.astream_events(**kwargs)
```

### Why

I think durability [defaults to](https://github.com/langchain-ai/langgraph/blob/076e2a3627206f5a1aef573aaca4a01e5af897ca/libs/langgraph/langgraph/pregel/main.py#L2676) `durability="async"` in LangGraph, which implies a lot of extra DB reads/write compared to`"exit"`.

I think it's useful to be able to control it as a lot of high traffic use cases will want to use `exit`.

### Proposed Solution

Not sure what is the best way to pass durability. I think there is no need to pass it at invocation, and could be set as an env var `AG_UI_LANGGRAPH_DURABILITY`.