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.

network/mhfpacket/msg_opcode_coverage_test.go

@@ -1,6 +1,7 @@
 package mhfpacket
 
 import (
+	"strings"
 	"testing"
 
 	"erupe-ce/common/byteframe"
@@ -261,7 +262,8 @@ func TestBuildCoverage_NotImplemented(t *testing.T) {
 				return
 			}
 			// Build returned an error, which is expected for NOT IMPLEMENTED stubs
-			if err.Error() != "NOT IMPLEMENTED" {
+			errMsg := err.Error()
+			if errMsg != "NOT IMPLEMENTED" && !strings.Contains(errMsg, "not implemented") {
 				t.Errorf("Build() returned unexpected error: %v", err)
 			}
 		})