Add test that showcases internal services can be registered

Related to #107

Author: kzu

src/DependencyInjection.Attributed.Tests/GenerationTests.cs

@@ -7,8 +7,18 @@
 
 namespace Tests.DependencyInjection;
 
-public record GenerationTests(ITestOutputHelper Output)
+public class GenerationTests(ITestOutputHelper Output)
 {
+    [Fact]
+    public void RegisterInternalService()
+    {
+        var collection = new ServiceCollection();
+        collection.AddServices();
+        var services = collection.BuildServiceProvider();
+
+        var instance = services.GetRequiredService<IService>();
+    }
+
     [Fact]
     public void RegisterSingletonService()
     {
@@ -335,4 +345,8 @@ public class KeyedByContractName { }
 public class DependencyFromKeyedContract([Import("contract")] KeyedByContractName dependency)
 {
     public KeyedByContractName Dependency => dependency;
-}
+}
+
+public interface IService { }
+[Service]
+class InternalService : IService { }