Codebase Indexing: Add Mistral as Embedding Provider for codestral-embed use

[jthegedus]

Mistral released codestral-embed and it would be good to use this in the Codebase Indexing feature.

Codestral Embed docs:

• https://docs.mistral.ai/capabilities/embeddings/code_embeddings/

Related:

• Indexing Related #4083
• [Feature Request] Add OpenAI Compatible embedder support for codebase indexing #4065
• Codebase Indexing: Add Google Gemini as provider #3967
• Feature Proposal: Allow Custom Embedding Model Configuration for Codebase Indexing #3959