refactor: Sprint 3b — remove dead code, non-functional provider, unused fields

refactor: use API-reported supports_vision instead of hardcoded model names

fix: recognize mistral-small-latest as multimodal (vision-capable)

fix(providers): désactiver vertex_api_key + ajouter OCR Mistral dédié

fix(mistral): refactoring structurel complet du provider Mistral

fix(mistralai): detect v0.x package + force pip --upgrade in Dockerfile

feat: multi-provider dynamique — ajout Mistral/Pixtral, détection auto des providers