Introduced unit tests for GetUnmatchedAttributes()

In a previous commit, we introduced a new `GetUnmatchedAttributes()` method to `TopicRepositoryBase` (2237a02), but neglected to include unit test coverage. As we prepare to extend this functionality, we're including the missing unit tests to ensure that we're not breaking existing functionality in the process.

This includes a "proxy" method on `StubTopicRepository` to publicly expose the `protected` `GetUnmatchedAttributes()` method via a `GetUnmatchedAttributesProxy()` passthrough.

Author: JeremyCaney

OnTopic.TestDoubles/StubTopicRepository.cs

@@ -171,6 +171,12 @@ public override void Move(Topic topic, Topic target, Topic? sibling) {
     public IEnumerable<AttributeValue> GetAttributesProxy(Topic topic, bool? isExtendedAttribute, bool? isDirty = null) =>
       base.GetAttributes(topic, isExtendedAttribute, isDirty);
 
+    /*==========================================================================================================================
+    | METHOD: GET UNMATCHED ATTRIBUTES (PROXY)
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <inheritdoc cref="TopicRepositoryBase.GetUnmatchedAttributes(Topic)" />
+    public IEnumerable<AttributeDescriptor> GetUnmatchedAttributesProxy(Topic topic) => base.GetUnmatchedAttributes(topic);
+
     /*==========================================================================================================================
     | METHOD: CREATE FAKE DATA
     \-------------------------------------------------------------------------------------------------------------------------*/

OnTopic.Tests/TopicRepositoryBaseTest.cs

@@ -106,6 +106,27 @@ public void GetAttributes_ExtendedAttributes_ReturnsExtendedAttributes() {
 
     }
 
+    /*==========================================================================================================================
+    | TEST: GET UNMATCHED ATTRIBUTES: RETURNS ATTRIBUTES
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <summary>
+    ///   Using <see cref="TopicRepositoryBase.GetUnmatchedAttributes(Topic)"/>, ensures that any attributes that exist on the
+    ///   <see cref="ContentTypeDescriptor"/> but not the <see cref="Topic"/> are returned.
+    /// </summary>
+    [TestMethod]
+    public void GetUnmatchedAttributes_ReturnsAttributes() {
+
+      var topic                 = TopicFactory.Create("Test", "ContentTypeDescriptor", 1);
+
+      topic.Attributes.SetValue("Title", "Title");
+
+      var attributes            = _topicRepository.GetUnmatchedAttributesProxy(topic);
+
+      Assert.IsTrue(attributes.Any());
+      Assert.IsFalse(attributes.Any(a => a.Key.Equals("Title", StringComparison.InvariantCultureIgnoreCase)));
+
+    }
+
     /*==========================================================================================================================
     | TEST: GET CONTENT TYPE DESCRIPTORS: RETURNS CONTENT TYPES
     \-------------------------------------------------------------------------------------------------------------------------*/