Function runChatParticipant

Drive one side of a goblin-chat interop exchange.

Resolves once all of the following have happened:

1. our send-message has been ack'd by the chatroom,
2. an inbound new-message carrying expectedRemoteMessage was observed on our subscription, and
3. an inbound new-message carrying localMessage was observed (the chatroom echoes our own send back to all subscribers, including ourselves — observing it confirms full round-trip).

Rejects if the deadline elapses, if subscribe returns a non-OK status, or if send-message rejects.