Add Mistral as a first-class provider alongside the 17 existing ones.
Mistral uses the OpenAI-compatible API at https://api.mistral.ai/v1
with provider-specific model prefix stripping (mistral/model → model).
Changes:
- Add Mistral to ProvidersConfig, IsEmpty(), HasProvidersConfig()
- Add mistral entry in default model_list (defaults.go)
- Add mistral protocol in factory_provider.go and getDefaultAPIBase()
- Add mistral prefix stripping in openai_compat normalizeModel()
- Add mistral case in legacy factory.go resolveProviderSelection()
- Add mistral migration entry in ConvertProvidersToModelList()
- Add mistral to supported providers in migrate/config.go
- Add mistral section in config.example.json
- Update AllProviders test (17 → 18 providers)
Tested end-to-end with mistral-small-latest model.
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