Fixed unit tests which evaluate missing interfaces in binding models

In binding models, the types used for topic references must implement `ITopicBindingModel` and the types used for relationships must implement `IAssociatedTopicBindingModel`. If they don't, an exception is thrown.

Previously, this was validated via unit tests that used `TopicViewModel` as the type for each of those, thus failing validation. We now derive `ITopicViewModel` from `ITopicBindingModel` (ffe775e) and `IAssociatedTopicBindingModel` (8ad42b9), however, and thus that actually satisfies the condition. That makes view models more flexible by allowing them to double as binding models. But it breaks our unit tests.

To fix this, I've introduced a new `EmptyViewModel` which implements no interfaces, and used it as the return type for the `InvalidreferenceTypeTopicBindingModel` as well as the `ContentTypes` collection type on the `InvalidRelationshipBaseTypeTopicBindingModel`. This effectively satisfies the expectations of those unit tests, and correctly returns them to throwing the expected exception.

Author: JeremyCaney

OnTopic.Tests/BindingModels/InvalidReferenceTypeTopicBindingModel.cs

@@ -4,9 +4,8 @@
 | Project       Topics Library
 \=============================================================================================================================*/
 using System;
-using OnTopic.Mapping.Annotations;
 using OnTopic.Models;
-using OnTopic.ViewModels;
+using OnTopic.Tests.ViewModels;
 
 namespace OnTopic.Tests.BindingModels {
 
@@ -24,7 +23,7 @@ public class InvalidReferenceTypeTopicBindingModel : BasicTopicBindingModel {
 
     public InvalidReferenceTypeTopicBindingModel(string? key = null) : base(key, "Page") { }
 
-    public TopicViewModel BaseTopic { get; } = new();
+    public EmptyViewModel BaseTopic { get; } = new();
 
   } //Class
 } //Namespace

OnTopic.Tests/BindingModels/InvalidRelationshipBaseTypeTopicBindingModel.cs

@@ -6,7 +6,7 @@
 using System;
 using System.Collections.ObjectModel;
 using OnTopic.Models;
-using OnTopic.ViewModels;
+using OnTopic.Tests.ViewModels;
 
 namespace OnTopic.Tests.BindingModels {
 
@@ -24,7 +24,7 @@ public class InvalidRelationshipBaseTypeTopicBindingModel : BasicTopicBindingMod
 
     public InvalidRelationshipBaseTypeTopicBindingModel(string? key = null) : base(key, "ContentTypeDescriptor") { }
 
-    public Collection<TopicViewModel> ContentTypes { get; } = new();
+    public Collection<EmptyViewModel> ContentTypes { get; } = new();
 
   } //Class
 } //Namespace

OnTopic.Tests/ViewModels/EmptyViewModel.cs

@@ -0,0 +1,23 @@
+/*==============================================================================================================================
+| Author        Ignia, LLC
+| Client        Ignia, LLC
+| Project       Topics Library
+\=============================================================================================================================*/
+using OnTopic.Models;
+
+namespace OnTopic.Tests.ViewModels {
+
+  /*============================================================================================================================
+  | VIEW MODEL: EMPTY
+  \---------------------------------------------------------------------------------------------------------------------------*/
+  /// <summary>
+  ///   A view model that does not implement any properties or interfaces. This will be invalid for mapping models that expect
+  ///   e.g. <see cref="IAssociatedTopicBindingModel"/> or <see cref="ITopicBindingModel"/>.
+  /// </summary>
+  /// <remarks>
+  ///   This is a sample class intended for test purposes only; it is not designed for use in a production environment.
+  /// </remarks>
+  public class EmptyViewModel {
+
+  }
+}