Added OpenAI Responses API as a new custom provider type, wiring `apiType: "responses"` through the core registry, engine routes, and dashboard UI with a dropdown selector; includes test coverage across the registry, routes, and component layers. Fusion-Task-Id: FN-5601 Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai> Fusion-Task-Id: FN-5601