Switch `MistralChatModel` to use mistral Python package

[jackmpcollins]

Currently MistralChatModel uses the openai Python package because the APIs were the same at one point. Now they have diverged in a few ways, so it might simplify things overall to switch to using the mistral Python package. https://github.com/mistralai/client-python/tree/main

Differences

• mistralai/client-python#78
• Mistral requires AssistantMessage after tool output https://github.com/jackmpcollins/magentic/pull/279/files#r1694354572
• #276