fix(fallback): honor explicit base_url and api_key config

[canderson26]

Summary

• pass fallback_model.base_url and explicit API key config through to resolve_provider_client()
• honor both direct api_key and api_key_env for custom/OpenAI-compatible fallback providers

Why

• fallback providers should use the explicit config the user set
• without this, custom fallback providers can silently fail by falling back to unrelated environment defaults

[teknium1]

Thanks for the contribution, @canderson26!

Closing this as a duplicate of #2587 (by @KiraKatana), which targets the same fix/feature. We're consolidating on that PR for review.

If you want to help push it over the line, please jump in there — or if you think your approach is better for a specific reason that isn't covered in the other PR, let us know and we can reopen.