[INSTRUMENTOR] Add support for Mistral AI

[Sigmabrogz]

Description

Add auto-instrumentation support for Mistral AI LLM API.

Why

Mistral is gaining popularity for its efficient models. Supporting it would expand our user base.

Implementation Guide

1. Create src/agentdbg/instrumentors/mistral_instrumentor.py
2. Wrap the Mistral client methods
3. Extract token counts from response
4. Add to auto_instrument() in __init__.py
5. Add tests and example

Resources

• Mistral Python SDK
• Reference: anthropic_instrumentor.py

Good First Issue

Follow the existing instrumentor pattern - straightforward implementation!