Fix WorkflowDefinitionActivityProvider

In addition to yielding defined outcomes as ports, the Root activity needs to be included as well in order for child activities to be visible to the activity walker.

src/modules/Elsa.Workflows.Management/Activities/WorkflowDefinitionActivity/WorkflowDefinitionActivityProvider.cs

@@ -57,6 +57,16 @@ public class WorkflowDefinitionActivityProvider : IActivityProvider
             IsBrowsable = true,
             Mode = PortMode.Port
         }).ToList();
+        
+        var rootPort = new Port
+        {
+            Name = nameof(WorkflowDefinitionActivity.Root),
+            DisplayName = "Root",
+            IsBrowsable = false,
+            Mode = PortMode.Embedded
+        };
+        
+        ports.Insert(0, rootPort);
 
         return new()
         {