[client] Support random wireguard port on client (#4085)

Adds support for using a random available WireGuard port when the user specifies port `0`.

- Updates `freePort` logic to bind to the requested port (including `0`) without falling back to the default.
- Removes default port assignment in the configuration path, allowing `0` to propagate.
- Adjusts tests to handle dynamically assigned ports when using `0`.

client/internal/connect.go

@@ -17,7 +17,6 @@ import (
 	"google.golang.org/grpc/codes"
 	gstatus "google.golang.org/grpc/status"
 
-	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/device"
 	"github.com/netbirdio/netbird/client/internal/dns"
 	"github.com/netbirdio/netbird/client/internal/listener"
@@ -526,17 +525,13 @@ func statusRecorderToSignalConnStateNotifier(statusRecorder *peer.Status) signal
 
 // freePort attempts to determine if the provided port is available, if not it will ask the system for a free port.
 func freePort(initPort int) (int, error) {
-	addr := net.UDPAddr{}
-	if initPort == 0 {
-		initPort = iface.DefaultWgPort
-	}
-
-	addr.Port = initPort
+	addr := net.UDPAddr{Port: initPort}
 
 	conn, err := net.ListenUDP("udp", &addr)
 	if err == nil {
+		returnPort := conn.LocalAddr().(*net.UDPAddr).Port
 		closeConnWithLog(conn)
-		return initPort, nil
+		return returnPort, nil
 	}
 
 	// if the port is already in use, ask the system for a free port