chore(deps): bump langchain-core (#13)

Bumps the pip group with 1 update in the /backend directory: [langchain-core](https://github.com/langchain-ai/langchain).


Updates `langchain-core` 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: direct:production
  dependency-group: pip
...

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Author: dependabot[bot]

backend/requirements.txt

@@ -11,7 +11,7 @@ python-multipart==0.0.22
 psycopg2-binary==2.9.11
 langchain==0.3.23
 langchain-openai==0.3.12
-langchain_core==1.2.22
+langchain_core==1.2.28
 langgraph>=0.2.0
 langfuse==2.60.2
 python-dotenv==1.1.0