partners: allow to set Prefix in AIMessage (for MistralAI)

[sdeparte]

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}

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1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
langchain	⬜️ Ignored (Inspect)	Visit Preview		Dec 20, 2024 3:41pm

[ccurme]

Thanks for this, can you add a test?

[ccurme]

Thank you!

Could be worth a follow-up adding this to the docs here, and/or the API reference (would probably require first fleshing out the Mistral API ref following the template here).