Feature request: support fast mode for openai provider

OpenAI supports fast mode. With oauth subscriptions would be very useful. especially with virtually unlimited pro subscription. Would be great to be able to enable this in MUX

<img width="437" height="142" alt="Image" src="https://github.com/user-attachments/assets/c3ac1fb8-073c-46f6-9da5-1b5140f30f1d" />