[dotnet] Support IPv6 to find free tcp port via DualMode

dotnet/src/webdriver/Internal/PortUtilities.cs

@@ -36,20 +36,18 @@ public static int FindFreePort()
         // Locate a free port on the local machine by binding a socket to
         // an IPEndPoint using IPAddress.Any and port 0. The socket will
         // select a free port.
-        int listeningPort = 0;
-        Socket portSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
         try
         {
-            IPEndPoint socketEndPoint = new IPEndPoint(IPAddress.Any, 0);
-            portSocket.Bind(socketEndPoint);
-            socketEndPoint = (IPEndPoint)portSocket.LocalEndPoint!;
-            listeningPort = socketEndPoint.Port;
+            using var socket = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
+            socket.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
+            return (socket.LocalEndPoint as IPEndPoint)!.Port;
         }
-        finally
+        catch (SocketException)
         {
-            portSocket.Close();
+            // If IPv6 is not supported, fallback to IPv4
+            using var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+            socket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+            return (socket.LocalEndPoint as IPEndPoint)!.Port;
         }
-
-        return listeningPort;
     }
 }