On a cache-cold interactive launch, a modelRoles.default pointing at a
models.yml discovery provider (openai-models-list) was silently replaced
by an unrelated authenticated provider's default. createAgentSession
resolves the default role before background discovery populates the
catalog, so the configured provider had no models yet and the fallback
fell through to pickDefaultAvailableModel.
Await one cache-aware discovery pass and re-resolve the configured
default whenever it is still unresolved (not only when nothing resolved
at all) before accepting a bundled-provider fallback.
Fixes#6162
roboomp