chore(deps)(deps): bump langchain-core from 1.2.22 to 1.2.28 in /backend

Bumps [langchain-core](https://github.com/langchain-ai/langchain) from 1.2.22 to 1.2.28.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain-core==1.2.22...langchain-core==1.2.28)

---
updated-dependencies:
- dependency-name: langchain-core
  dependency-version: 1.2.28
  dependency-type: indirect
...

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Author: dependabot[bot]