AllowNatTraversal is properly set for stopped socket (dotnet#32603)

If AllowNatTraversal is called on a stopped TcpListener when ```_serverSocket``` is null, ```allow``` value gets stored in a field. Subsequently, if a new socket is created on Start call, the saved value is read and the corresponding argument is passed to SetIPProtectionLevel call after which the value is reset to preserve the current IPProtectionLevel propagation logic. The follow-up issue is dotnet#32653 
Fixes dotnet#32551

src/libraries/System.Net.Sockets/src/System/Net/Sockets/TCPListener.cs

@@ -15,6 +15,7 @@ public class TcpListener
         private Socket _serverSocket;
         private bool _active;
         private bool _exclusiveAddressUse;
+        private bool? _allowNatTraversal;
 
         // Initializes a new instance of the TcpListener class with the specified local end point.
         public TcpListener(IPEndPoint localEP)
@@ -118,7 +119,14 @@ public void AllowNatTraversal(bool allowed)
                 throw new InvalidOperationException(SR.net_tcplistener_mustbestopped);
             }
 
-            _serverSocket.SetIPProtectionLevel(allowed ? IPProtectionLevel.Unrestricted : IPProtectionLevel.EdgeRestricted);
+            if (_serverSocket != null)
+            {
+                SetIPProtectionLevel(allowed); // Set it only for the current socket to preserve existing behavior
+            }
+            else
+            {
+                _allowNatTraversal = allowed;
+            }
         }
 
         // Starts listening to network requests.
@@ -335,6 +343,9 @@ public static TcpListener Create(int port)
             return listener;
         }
 
+        private void SetIPProtectionLevel(bool allowed)
+            => _serverSocket.SetIPProtectionLevel(allowed ? IPProtectionLevel.Unrestricted : IPProtectionLevel.EdgeRestricted);
+
         private void CreateNewSocketIfNeeded()
         {
             _serverSocket ??= new Socket(_serverSocketEP.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
@@ -343,6 +354,12 @@ private void CreateNewSocketIfNeeded()
             {
                 _serverSocket.ExclusiveAddressUse = true;
             }
+
+            if (_allowNatTraversal != null)
+            {
+                SetIPProtectionLevel(_allowNatTraversal.GetValueOrDefault());
+                _allowNatTraversal = null; // Reset value to avoid affecting more sockets
+            }
         }
     }
 }

src/libraries/System.Net.Sockets/tests/FunctionalTests/TcpListenerTest.cs

@@ -44,6 +44,36 @@ public void Active_TrueWhileRunning(int ctor)
             Assert.False(listener.Active);
         }
 
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void AllowNatTraversal_NotStarted_SetSuccessfully()
+        {
+            var listener = new TcpListener(IPAddress.Loopback, 0);
+            listener.AllowNatTraversal(true);
+            listener.Start();
+            listener.Stop();
+        }
+
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void AllowNatTraversal_Started_ThrowsException()
+        {
+            var listener = new TcpListener(IPAddress.Loopback, 0);
+            listener.Start();
+            Assert.Throws<InvalidOperationException>(() => listener.AllowNatTraversal(true));
+            listener.Stop();
+        }
+
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void AllowNatTraversal_StartedAndStopped_SetSuccessfully()
+        {
+            var listener = new TcpListener(IPAddress.Loopback, 0);
+            listener.Start();
+            listener.Stop();
+            listener.AllowNatTraversal(true);
+        }
+
         [Fact]
         public void Start_InvalidArgs_Throws()
         {