chore(deps): bump langchain-core in /python/assistant-stream

Bumps [langchain-core](https://github.com/langchain-ai/langchain) from 0.3.68 to 0.3.84.
- [Release notes](https://github.com/langchain-ai/langchain/releases)
- [Commits](langchain-ai/langchain@langchain-core==0.3.68...langchain-core==0.3.84)

---
updated-dependencies:
- dependency-name: langchain-core
  dependency-version: 0.3.84
  dependency-type: direct:production
...

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Author: dependabot[bot]