build: bump autogen-agentchat in /src/backend (#16)

Bumps [autogen-agentchat](https://github.com/microsoft/autogen) from 0.4.0dev1 to 0.4.0.dev12.
- [Release notes](https://github.com/microsoft/autogen/releases)
- [Commits](microsoft/autogen@v0.4.0dev1...v0.4.0.dev12)

---
updated-dependencies:
- dependency-name: autogen-agentchat
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/backend/requirements.txt

@@ -1,6 +1,6 @@
 fastapi
 uvicorn
-autogen-agentchat==0.4.0dev1
+autogen-agentchat==0.4.0.dev12
 azure-cosmos
 azure-identity
 python-dotenv