feat: bridge mixed wire protocol SUDP payloads (#5347)

SUDP payload codec follows transport wireProtocol; same-protocol v1/v1 and v2/v2 keep raw join; only mixed proxy/visitor protocols use message-aware bridge; no new capability/selection field.

server/visitor/visitor_test.go

@@ -0,0 +1,61 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package visitor
+
+import (
+	"net"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/fatedier/frp/pkg/proto/wire"
+	"github.com/fatedier/frp/pkg/util/util"
+)
+
+func TestManagerNewConnCarriesWireProtocol(t *testing.T) {
+	vm := NewManager()
+	listener, err := vm.Listen("sudp", "secret", []string{"*"})
+	require.NoError(t, err)
+	defer listener.Close()
+
+	client, server := net.Pipe()
+	defer client.Close()
+	defer server.Close()
+
+	now := time.Now().Unix()
+	errCh := make(chan error, 1)
+	go func() {
+		errCh <- vm.NewConn(
+			"sudp",
+			server,
+			now,
+			util.GetAuthKey("secret", now),
+			false,
+			false,
+			"user",
+			wire.ProtocolV2,
+		)
+	}()
+
+	acceptedConn, err := listener.Accept()
+	require.NoError(t, err)
+	defer acceptedConn.Close()
+
+	getter, ok := acceptedConn.(interface{ WireProtocol() string })
+	require.True(t, ok)
+	require.Equal(t, wire.ProtocolV2, getter.WireProtocol())
+	require.NoError(t, <-errCh)
+}