FIX: Serialize parent categories first (#26294)

When categories are loaded by the frontend, the parent category is
looked up by ID and the `parentCategory` is set with the result. If the
categories returned are not in order, the parent category may miss.

app/models/topic_list.rb

@@ -77,7 +77,7 @@ class TopicList
 
   def categories
     @categories ||=
-      topics.map { |t| [t.category&.parent_category, t.category] }.uniq.flatten.compact
+      topics.map { |t| [t.category&.parent_category, t.category] }.flatten.uniq.compact
   end
 
   def load_topics

lib/topic_view.rb

@@ -727,9 +727,9 @@ class TopicView
   end
 
   def categories
-    categories = [category, category&.parent_category]
-    categories += suggested_topics.categories if suggested_topics
-    categories.compact.uniq
+    @categories ||= [category&.parent_category, category, suggested_topics&.categories].flatten
+      .uniq
+      .compact
   end
 
   protected