Allow to change the provider when key are not present.

[jeanlaurent]

If I got an OPENAI_API_KEY but no ANTHROPIC one, then if using a yaml file which use claude this will fail.
It hurts discovery

-> Maybe it's a fallback flag, maybe this is a gateway feature.

[krissetto]

I think we can consider this to be completed by #437.

I'll close this issue for now because I don't think we want to silently use another model/provider without an explicit user override, and cagent new already has this "use best available provider based on available api keys" logic.

Feel free to re-open if necessary

[rumpl]

@krissetto I agree that we should close this one, but are you sure it's #43?

[krissetto]

@rumpl LOL good catch i missed a number, its #437