Add test for middleware evaluation with IfNotRegistered

This test verifies that middleware is evaluated correctly when using IfNotRegistered with a service descriptor. It ensures the middleware is invoked during the resolution pipeline as expected.

test/Autofac.Specification.Test/Registration/RegistrationOnlyIfTests.cs

@@ -143,6 +143,27 @@ public void IfNotRegistered_EvaluatesServiceMiddleware()
         Assert.True(middlewareInvoked);
     }
 
+    [Fact]
+    public void IfNotRegistered_EvaluatesServiceMiddleware_WithDescriptor()
+    {
+        var builder = new ContainerBuilder();
+        var middlewareInvoked = false;
+
+        builder.RegisterType<ServiceA>().As<IService>().IfNotRegistered(typeof(IService));
+
+        const string descriptor = "custom-middleware";
+        builder.RegisterServiceMiddleware(typeof(IService), descriptor, PipelinePhase.ResolveRequestStart, (context, next) =>
+            {
+                next(context);
+                middlewareInvoked = true;
+            });
+
+        var container = builder.Build();
+
+        var result = container.Resolve<IService>();
+        Assert.True(middlewareInvoked);
+    }
+
     [Fact]
     public void IfNotRegistered_CanBeDecoratedByModuleWhenModuleRegistered1st()
     {