Add support for Mistral models

[nico-hoff]

• Updated .env.example to include Mistral API key configuration.
• Modified frontend model interfaces to recognize Mistral as a provider.
• Enhanced API model definitions to include Mistral models.
• Updated model retrieval logic to handle Mistral-specific API key requirements.
• Updated poetry.lock and pyproject.toml to include langchain-mistralai dependency.