fix: correct Play ClientBound plugin.Message packet ID for 1.21.9+ (fixes minekube#612) (minekube#674)

The plugin.Message packet was incorrectly mapped to 0x1C for Minecraft
1.21.9 instead of staying at 0x18 (same as 1.21.5). This caused the
proxy to not recognize BungeeCord plugin channel messages from backend
servers, silently breaking server transfers via the "Connect" subchannel.

Author: robinbraemer

pkg/edition/java/proto/state/register.go

@@ -523,7 +523,6 @@ func init() {
 		m(0x18, version.Minecraft_1_20_2),
 		m(0x19, version.Minecraft_1_20_5),
 		m(0x18, version.Minecraft_1_21_5),
-		m(0x1C, version.Minecraft_1_21_9),
 	)
 	Play.ClientBound.Register(&p.RemoveResourcePack{},
 		m(0x43, version.Minecraft_1_20_3),

pkg/edition/java/proto/state/register_test.go

@@ -0,0 +1,37 @@
+package state
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.minekube.com/gate/pkg/edition/java/proto/packet/plugin"
+	"go.minekube.com/gate/pkg/edition/java/proto/version"
+	proto "go.minekube.com/gate/pkg/gate/proto"
+)
+
+// TestPluginMessagePacketID_1_21_9 verifies that the Play ClientBound plugin.Message
+// packet ID is correct for Minecraft 1.21.9 (protocol 773).
+// This was incorrectly mapped to 0x1C instead of 0x18, causing BungeeCord
+// plugin channel messages to be silently dropped (fixes #612).
+func TestPluginMessagePacketID_1_21_9(t *testing.T) {
+	tests := []struct {
+		name     string
+		protocol proto.Protocol
+		wantID   int
+	}{
+		{"1.21.5 should be 0x18", version.Minecraft_1_21_5.Protocol, 0x18},
+		{"1.21.7 should be 0x18", version.Minecraft_1_21_7.Protocol, 0x18},
+		{"1.21.9 should be 0x18", version.Minecraft_1_21_9.Protocol, 0x18},
+		{"1.21.11 should be 0x18", version.Minecraft_1_21_11.Protocol, 0x18},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			reg := Play.ClientBound.ProtocolRegistry(tc.protocol)
+			require.NotNil(t, reg, "no protocol registry for protocol %d", tc.protocol)
+			id, ok := reg.PacketID(&plugin.Message{})
+			require.True(t, ok, "plugin.Message not registered for protocol %d", tc.protocol)
+			require.Equal(t, proto.PacketID(tc.wantID), id,
+				"plugin.Message packet ID mismatch for protocol %d", tc.protocol)
+		})
+	}
+}