chore(deps)(deps): bump langchain from 1.2.13 to 1.2.15 in /backend

Bumps [langchain](https://github.com/langchain-ai/langchain) from 1.2.13 to 1.2.15.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain==1.2.13...langchain==1.2.15)

---
updated-dependencies:
- dependency-name: langchain
  dependency-version: 1.2.15
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]