add native Mistral LLM provider

[yai-dev]

Summary

Add a native Mistral provider to @agentrail/core.

Important Context

Mistral is already usable today through the existing openai provider plus a baseUrl override. This issue is therefore about native ergonomics and Mistral-specific behavior, not basic compatibility.

Proposed Implementation

1. Add the Mistral SDK to packages/core/package.json
2. Implement a native provider in packages/core/src/llm/providers/
3. Register/export it through the current providers index
4. Add tests against the current core provider test setup

Acceptance Criteria

• provider: "mistral" works in ModelConfig
• Text streaming works
• Tool calling works
• baseUrl override is respected for self-hosted / alternate deployments if applicable
• Mistral-specific behavior has a clear home separate from generic OpenAI compatibility
• Tests pass
• Changeset included