Extended Refresh() with data validation for root content type

The initial version didn't validate that the `ContentTypeDescriptor` was, in fact, the root `ContentTypeDescriptor`—i.e., the one sourced from `Root:Configuration:ContentTypes`. This addresses that by adding in a `Contract.Requires()` call.

Author: JeremyCaney

OnTopic/Metadata/ContentTypeDescriptorCollection.cs

@@ -53,6 +53,12 @@ public void Refresh(ContentTypeDescriptor? rootContentType) {
         return;
       }
 
+      Contract.Requires(
+        rootContentType.Key.Equals("ContentTypes", StringComparison.OrdinalIgnoreCase),
+        $"The {nameof(rootContentType)} is expected to represent the root of the content type topic graph, with a " +
+        $"key of 'ContentTypes'. Instead, the supplied ContentTypeDescriptor has a key of '{rootContentType.Key}'."
+      );
+
       /*------------------------------------------------------------------------------------------------------------------------
       | Clear any existing values
       \-----------------------------------------------------------------------------------------------------------------------*/