chore(deps): bump langgraph-sdk from 0.3.12 to 0.3.15

Bumps [langgraph-sdk](https://github.com/langchain-ai/langgraph) from 0.3.12 to 0.3.15.
- [Release notes](https://github.com/langchain-ai/langgraph/releases)
- [Commits](langchain-ai/langgraph@0.3.12...0.3.15)

---
updated-dependencies:
- dependency-name: langgraph-sdk
  dependency-version: 0.3.15
  dependency-type: indirect
...

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Author: dependabot[bot]