Error responses from the websocket multiplexer were missing the
request ID and using a bare string format instead of the structured
error protocol. This caused clients to hang when a request failed
(e.g. unsupported service for a flow) because the error could not
be routed to the waiting caller.
Include request ID in all error paths, use structured error format
({message, type}) with complete flag, and extract the ID early in
receive() so even malformed requests get a routable error when
possible.
Updated tests - tests were coded against invalid protocol messages
