OptionalExpectOptional. Registers the per-call media-socket token for a call MJ has just placed (the REST
calls.create response is the first moment the Call SID is known). The pump uses it to authenticate
the Media-Streams start frame; a pump that does not authenticate sockets may omit it.
Registers the call's stream-SID resolver, so outbound frames address the right stream.
Registers the inbound-frame handler for the call's websocket (latest handler wins).
Sends one outbound Media-Streams frame on the call's websocket.
The bidirectional Media-Streams frame pump for a single call's websocket. Production wires this over the accepted websocket for the Call SID; tests inject a fake. The pump speaks raw frame objects — transcode happens in RealTwilioBindings via the pure helpers above.