Bump langchain-text-splitters in /integrations/langgraph

Bumps [langchain-text-splitters](https://github.com/langchain-ai/langchain) from 0.3.8 to 0.3.9.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain-text-splitters==0.3.8...langchain-text-splitters==0.3.9)

---
updated-dependencies:
- dependency-name: langchain-text-splitters
  dependency-version: 0.3.9
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]