Previously, Prometheus always used params.currentModel (UI-selected model)
when no explicit configuration existed. This bypassed the intended fallback
chain (claude-opus-4-6 → gpt-5.4 → glm-5 → gemini-3.1-pro).
Now, the UI-selected model is only used if it matches one of the models
in Prometheus's fallback chain. Otherwise, the fallback chain resolution
takes over and finds an available model.
Fixes#2986