[client] Fix IPv6 address formatting in DNS address construction (#5603)

Replace fmt.Sprintf("%s:%d", ip, port) with net.JoinHostPort() to
properly handle IPv6 addresses that need bracket wrapping (e.g.,
[2606:4700:4700::1111]:53 instead of 2606:4700:4700::1111:53).

Without this fix, configuring IPv6 nameservers causes "too many colons
in address" errors because Go's net.Dial cannot parse the malformed
address string.

Fixes #5601
Related to #4074

Co-authored-by: easonysliu <easonysliu@tencent.com>

client/internal/routemanager/client/client.go

@@ -3,7 +3,9 @@ package client
 import (
 	"context"
 	"fmt"
+	"net"
 	"reflect"
+	"strconv"
 	"time"
 
 	log "github.com/sirupsen/logrus"
@@ -564,7 +566,7 @@ func HandlerFromRoute(params common.HandlerParams) RouteHandler {
 		return dnsinterceptor.New(params)
 	case handlerTypeDynamic:
 		dns := nbdns.NewServiceViaMemory(params.WgInterface)
-		dnsAddr := fmt.Sprintf("%s:%d", dns.RuntimeIP(), dns.RuntimePort())
+		dnsAddr := net.JoinHostPort(dns.RuntimeIP().String(), strconv.Itoa(dns.RuntimePort()))
 		return dynamic.NewRoute(params, dnsAddr)
 	default:
 		return static.NewRoute(params)