chore(deps): bump langgraph-checkpoint in /samples/mcp-functions-agent

Bumps [langgraph-checkpoint](https://github.com/langchain-ai/langgraph) from 2.1.1 to 4.0.0.
- [Release notes](https://github.com/langchain-ai/langgraph/releases)
- [Commits](langchain-ai/langgraph@checkpoint==2.1.1...checkpoint==4.0.0)

---
updated-dependencies:
- dependency-name: langgraph-checkpoint
  dependency-version: 4.0.0
  dependency-type: indirect
...

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Author: dependabot[bot]