Consolidated the content type validation logic

Previously, the `contentType` (now `contentTypeDescriptor`) was defined in one block, but validated in a later block. These are now brought together. In addition, the `GetContentTypeDescriptors()` collection is confirmed to contain the content type before retrieving it. Otherwise, the validation will never fire, and instead a less-intuitive `IndexOutOfRangeException` will be thrown.

It's worth noting that we no longer _need_ the `ContentTypeDescriptor` here. Instead, this is needed by the new `GetAttributes()` and `GetNullableAttributes()` methods on `TopicRepositoryBase` (see #6662f07, #2237a02), which call `GetContentTypes()` on their own. To reduce the number of times this needs to be looked up and validated, it may make more sense to just relay this to `GetAttributes()` and `GetNullableAttributes()` in the future; we'll need to consider that.

Author: JeremyCaney

OnTopic.Data.Sql/SqlTopicRepository.cs

@@ -15,6 +15,7 @@
 using Microsoft.Data.SqlClient;
 using OnTopic.Attributes;
 using OnTopic.Internal.Diagnostics;
+using OnTopic.Metadata;
 using OnTopic.Repositories;
 
 namespace OnTopic.Data.Sql {
@@ -659,7 +660,14 @@ public override int Save([NotNull]Topic topic, bool isRecursive = false, bool is
       /*------------------------------------------------------------------------------------------------------------------------
       | Validate content type
       \-----------------------------------------------------------------------------------------------------------------------*/
-      var contentType = GetContentTypeDescriptors()[topic.Attributes.GetValue("ContentType", "Page")?? "Page"];
+      var contentTypes          = GetContentTypeDescriptors();
+      var contentType           = topic.Attributes.GetValue("ContentType", "Page")?? "Page";
+      var contentTypeDescriptor = contentTypes.GetTopic(contentType) as ContentTypeDescriptor;
+
+      Contract.Assume(
+        contentTypeDescriptor,
+        $"The Topics repository or database does not contain a ContentTypeDescriptor for the {contentType} content type."
+      );
 
       /*------------------------------------------------------------------------------------------------------------------------
       | Establish attribute containers with schema
@@ -678,11 +686,6 @@ public override int Save([NotNull]Topic topic, bool isRecursive = false, bool is
         }
       );
 
-      Contract.Assume(
-        contentType,
-        $"The Topics repository or database does not contain a ContentTypeDescriptor for the {contentType} content type."
-      );
-
       /*------------------------------------------------------------------------------------------------------------------------
       | Add indexed attributes that are dirty
       \-----------------------------------------------------------------------------------------------------------------------*/