#14730: Add Mistral AI as a model provider

This PR adds support for Mistral AI as a model provider in the onboarding flow. It includes the necessary configuration for Mistral's API, model definitions, and integration into the wizard. Programmed with the help of Gemini CLI. I know how the code does <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR wires Mistral AI into the onboarding/auth flow by adding a new `mistral-api-key` auth choice, a `--mistral-api-key` CLI flag, and provider/model configuration defaults (base URL, default model definition, and provider config application). The main issues are (1) a TypeScript type mismatch in `auth-choice-options.ts` where the new `mistral` group is not included in the local `AuthChoiceGroupId` union, and (2) `setMistralApiKey` is synchronous but is awaited in the Mistral auth-choice handler, which may fail lint/type checks and is inconsistent with other credential setters. <h3>Confidence Score: 3/5</h3> - This PR is close to mergeable but likely fails typecheck/lint as-is. - Confidence reduced due to a confirmed TypeScript type mismatch (missing "mistral" in `AuthChoiceGroupId`) and awaiting a non-async function, which commonly fails repo lint/type rules. Otherwise changes are localized to onboarding/provider wiring. - src/commands/auth-choice-options.ts, src/commands/auth-choice.apply.api-providers.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->