feat(backend): add SIMPLE_STREAMING WebRTC control-path streaming

Backend/media/providers/mock.ts

@@ -37,6 +37,8 @@ export class MockMediaProvider implements MediaProvider {
   name = 'mock';
 
   async createSession(input: MediaSessionCreateInput): Promise<MediaSessionCreateResult> {
+    // SIMPLE_STREAMING bypasses provider-backed transport at runtime. This metadata
+    // path is kept only for legacy endpoints and backwards compatibility.
     const mediaSessionId = `mock_${input.streamSessionId}`;
     const baseUrl = getBaseUrl();