fix(langgraph-api): stop wiping graph recursionLimit with undefined (#2645)

When a run omitted `config.recursion_limit`, we still passed
`recursionLimit: undefined` into `streamEvents`. Pregel merges that over
the graph's `withConfig` default, so agents (including deepagents with a
bound 10000) fell back to langchain-core's 25 and hit
`GraphRecursionError` early.

Omit undefined keys from stream/assistant config objects so bound limits
stick.

## Test plan
- [x] `pnpm test tests/stream.test.mts` in `libs/langgraph-api` (8
passed)
- [x] Changeset added for affected package(s)