feat: add support for the model Codestral to power the feature explain-code

What does this merge request do and why?

• Add support for Codestral to power explain-code.

• Eval results https://smith.langchain.com/o/477de7ad-583e-47b6-a1c4-c4a0300e7aca/datasets/3bc706a4-36b1-4a89-8f48-561e0555d4ec/compare?selectedSessions=b3609811-707a-478c-933d-784d6fa3511c&baseline=undefined.

• Part of Evaluate Codestral for Chat (gitlab-org/gitlab#577712 - closed).

How to set up and validate locally

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Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.