feat: complete TASK-WP0-07 supervisor

supervisor/Dada.Supervisor/LoopbackPortGuard.cs

@@ -0,0 +1,21 @@
+using System.Net;
+using System.Net.Sockets;
+
+namespace Dada.Supervisor;
+
+internal static class LoopbackPortGuard
+{
+    internal static SupervisorState Check()
+    {
+        try
+        {
+            using var listener = new TcpListener(IPAddress.Parse(LoopbackEndpoint.Host), LoopbackEndpoint.Port);
+            listener.Start();
+            return SupervisorState.Starting;
+        }
+        catch (SocketException)
+        {
+            return SupervisorState.PortInUse;
+        }
+    }
+}