Fallback SBD Messaging Management, Events, and Testing

The only path to entering sbd-fallback-messaging-mode is that we get an error establishing a webrtc connection or sending the first webrtc message.

Write tests that ensure the following:
1. if the first message fails to send, it is sent again over the fallback messaging method
1. if we receive a fallback message (and have not successfully sent one of our own over webrtc) we also switch to fallback, aborting trying to set up webrtc
1. a "Disconnected" event is NOT emitted when we switch to fallback mode
1. if we get any kind of error after the first successful send of a message over webrtc, we do NOT enter fallback mode, but instead the whole connection is closed and a "Disconnected" event is emitted

Locate the SBD fallback messaging and WebRTC connection setup entry points, then trace how the first send, incoming fallback messages, errors, and connection closure are handled. Add tests covering the four listed transitions and event outcomes, and run the relevant Rust test suite to verify fallback retries only before a successful WebRTC send.

Written by the indexing model from the issue text.