Way to set fallback model on limits hit

[bushig]

What would you like to be added?

Add ENV variable similar to OPENAI_MODEL to change fallback model when main model is exausted.

For example OPENAI_FALLBACK_MODEL

Why is this needed?

At the moment if selected model hits limits (429 api response), session falls back to gemini-2.5-flash. And there is no way to change fallback model to something like qwen-flash-coder.

Additional context

Possible quota limitations in place or slow response times detected. Switching to the gemini-2.5-flash model for the rest of this session.