When a runtime-specific entry point must use a provider proxy, make the proxy builder

General lesson: When a runtime-specific entry point must use a provider proxy, make the proxy builder a fail-closed boundary that validates its URL and credential together before service construction. Test that builder directly with each setting blank, so changing global provider-selection defaults remains unnecessary. Applicability: this protects explicit LiveKit-style routing; malformed endpoint syntax may still be validated by deployment readiness checks.