fix(channelserver): send ACK on empty Raviente register payload

handleMsgSysOperateRegister returned without sending an ACK when the
payload was empty, causing the client to softlock waiting for a response.
Send doAckBufSucceed with nil data on the early-return path to match
the success-path ACK type.

Also update tests to expect error returns instead of panics from
unimplemented Build/Parse stubs (matching prior panic→error refactor),
and mark resolved anti-patterns in docs.

server/channelserver/handlers_register.go

@@ -59,6 +59,7 @@ func handleMsgSysOperateRegister(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgSysOperateRegister)
 
 	if len(pkt.RawDataPayload) == 0 {
+		doAckBufSucceed(s, pkt.AckHandle, nil)
 		return
 	}