feat: Add Mistral AI model support through OpenAIChatCompletionClient

[Ricardo-M-L]

Summary

Adds native Mistral AI model support to OpenAIChatCompletionClient, enabling seamless integration with Mistral's OpenAI-compatible API.

Changes:

• _model_info.py: Add model entries for all Mistral AI model families (mistral-large, mistral-medium, mistral-small, codestral, open-codestral-mamba, pixtral, ministral, open-mistral-nemo) including model pointers (-latest aliases), model capabilities, and token limits
• _openai_client.py: Auto-detect Mistral models and configure base_url (https://api.mistral.ai/v1/), MISTRAL_API_KEY env var, and default include_name_in_message=False to prevent HTTP 422 errors from extra name field in messages
• Tests: 11 new test functions covering model detection, info retrieval, token limits, pointer resolution, auto base URL, auto API key, and include_name_in_message behavior

Usage:

# Minimal setup - just set MISTRAL_API_KEY env var
client = OpenAIChatCompletionClient(model="mistral-small-latest")

# Or pass API key directly
client = OpenAIChatCompletionClient(model="mistral-large-latest", api_key="your-key")

Fixes #6151
Closes #6147

Test plan

• test_openai_chat_completion_client_with_mistral_model - basic instantiation
• test_openai_chat_completion_client_with_mistral_model_variants - all model variants
• test_is_mistral_model - prefix detection (positive & negative cases)
• test_mistral_model_info - capability flags per model family
• test_mistral_model_token_limits - token limits for all models
• test_mistral_resolve_model - pointer resolution (latest -> versioned)
• test_mistral_auto_base_url - automatic base URL configuration
• test_mistral_auto_api_key - MISTRAL_API_KEY env var detection
• test_mistral_include_name_disabled_by_default - fixes Update of @langchain/mistralai necessary for using Mistral API in Autogen Studio #6147 HTTP 422
• test_mistral_include_name_can_be_overridden - explicit override works

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