Add Mistral model support to OpenAIChatCompletionClient

[he-yufeng]

Summary

• Add Mistral AI model entries to _model_info.py: model pointers (aliases), model info (capabilities), and token limits for mistral-large-2411, mistral-small-2503, codestral-2501, pixtral-large-2411, pixtral-12b-2409, open-mistral-nemo-2407, ministral-8b-2410, open-codestral-mamba, and mistral-embed
• Auto-detect base_url (https://api.mistral.ai/v1/) and MISTRAL_API_KEY environment variable in OpenAIChatCompletionClient, following the existing pattern for Gemini, Anthropic, and Llama models
• Add is_mistral_model() helper using prefix-based matching for all Mistral model name prefixes

Motivation

Currently, using Mistral models requires manually specifying base_url and api_key. This PR adds the same level of auto-detection already available for Gemini, Anthropic, and Llama models, allowing users to simply write:

client = OpenAIChatCompletionClient(model="mistral-large-latest")

Test plan

• Verify mistral-large-latest resolves to mistral-large-2411 via model pointers
• Verify OpenAIChatCompletionClient(model="mistral-large-latest") auto-sets base_url and reads MISTRAL_API_KEY
• Verify existing Gemini/Claude/Llama auto-detection is unaffected

Closes #6151

🤖 Generated with Claude Code

[he-yufeng]

Hey, any chance this can get merged? It's been approved for a couple weeks now.

[he-yufeng]

@anandfresh any chance you could merge this? Been approved since March.

[he-yufeng]

gentle bump — this has been approved for a while now, anything needed from my side to get it across the line?

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