chore(deps): Bump langgraph from 1.1.2 to 1.2.6

Bumps [langgraph](https://github.com/langchain-ai/langgraph) from 1.1.2 to 1.2.6.
- [Release notes](https://github.com/langchain-ai/langgraph/releases)
- [Commits](langchain-ai/langgraph@1.1.2...1.2.6)

---
updated-dependencies:
- dependency-name: langgraph
  dependency-version: 1.2.2
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]