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.
