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fcba567a - partners: allow to set Prefix in AIMessage (for MistralAI) (#28846)

partners: allow to set Prefix in AIMessage (for MistralAI) (#28846) **Description:** Added ability to set `prefix` attribute to prevent error : ``` httpx.HTTPStatusError: Error response 400 while fetching https://api.mistral.ai/v1/chat/completions: {"object":"error","message":"Expected last role User or Tool (or Assistant with prefix True) for serving but got assistant","type":"invalid_request_error","param":null,"code":null} ``` Co-authored-by: Sylvain DEPARTE <sylvain.departe@wizbii.com>