GitHub Copilot adds Project HydraFusion and new models to model picker

GitHub has made the Project HydraFusion research preview available in the GitHub Copilot app and @code, where it orchestrates multiple models rather than acting as a single model. New models from Anthropic (Fable 5.1 and Opus 5.5) and OpenAI (GPT-6.1 Sol) are also now selectable in the Copilot model picker.