ax(node): add missing _Bad and _Ugly tests for TestMessage_Reply

AX Principle 10 requires all three categories (Good, Bad, Ugly) for
every test function. TestMessage_Reply had only _Good; adds _Bad
(unmarshalable payload propagates error) and _Ugly (self-to-self
addressing preserves correct From/To reversal).

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/message_test.go

@@ -89,6 +89,51 @@ func TestMessage_Reply_Good(t *testing.T) {
 	}
 }
 
+// TestMessage_Reply_Bad verifies that Reply returns an error when the payload cannot be marshalled.
+//
+//	original, _ := NewMessage(MsgPing, "sender", "receiver", nil)
+//	_, err := original.Reply(MsgPong, make(chan int)) // unmarshalable type
+//	if err == nil { t.Error("expected error for unmarshalable payload") }
+func TestMessage_Reply_Bad(t *testing.T) {
+	original, err := NewMessage(MsgPing, "sender", "receiver", nil)
+	if err != nil {
+		t.Fatalf("failed to create original message: %v", err)
+	}
+
+	// A channel cannot be marshalled to JSON — Reply must propagate the error.
+	_, replyErr := original.Reply(MsgPong, make(chan int))
+	if replyErr == nil {
+		t.Error("expected error when payload cannot be marshalled, got nil")
+	}
+}
+
+// TestMessage_Reply_Ugly verifies that Reply correctly reverses From/To even when both are identical.
+//
+//	original, _ := NewMessage(MsgPing, "self", "self", nil)
+//	reply, err := original.Reply(MsgPong, nil)
+//	if reply.From != "self" || reply.To != "self" { t.Error("from/to should both be 'self'") }
+func TestMessage_Reply_Ugly(t *testing.T) {
+	original, err := NewMessage(MsgPing, "self", "self", nil)
+	if err != nil {
+		t.Fatalf("failed to create original message: %v", err)
+	}
+
+	reply, replyErr := original.Reply(MsgPong, nil)
+	if replyErr != nil {
+		t.Fatalf("unexpected error: %v", replyErr)
+	}
+
+	if reply.From != "self" {
+		t.Errorf("expected From 'self', got '%s'", reply.From)
+	}
+	if reply.To != "self" {
+		t.Errorf("expected To 'self', got '%s'", reply.To)
+	}
+	if reply.ReplyTo != original.ID {
+		t.Errorf("expected ReplyTo '%s', got '%s'", original.ID, reply.ReplyTo)
+	}
+}
+
 func TestMessage_ParsePayload_Good(t *testing.T) {
 	t.Run("ValidPayload", func(t *testing.T) {
 		payload := StartMinerPayload{