fix append_message

[yiranwu0]

Handle error: InvalidRequestError: Invalid parameter: only messages with role 'assistant' can have a function call.

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• [ ] I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• [ ] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [ ] I've made sure all auto checks have passed.

[sonichi]

Please make PR in the new repo https://github.com/microsoft/autogen