When moving a voice agent to a new transport, audit every media room and every later

When moving a voice agent to a new transport, audit every media room and every later provider switch, not just the first pipeline constructor. A direct-provider escape can survive in consultation media or a flow action even if the initial STT/LLM/TTS services use a proxy. Keep the transport-specific requirement explicit, pin dynamic LLM reconfiguration to the proxy endpoint, and fail closed when proxy settings are absent. This applies to agents with multiple rooms or per-node model routing; it does not prove runtime success without a deployed media test.