PERF: Ensure suggested topics is only loaded on last page of topic view (#28507)

This commit improves `TopicsController#show` to not load suggested and
related topics unless it is the last page of the topic's view.
Previously, we avoided loading suggested and related topics by the use
of conditionals in the `TopicViewSerializer` to avoid calling
`TopicView#suggested_topics` and `TopicView#related_topics`. However,
this pattern is not reliable as the methods can still be called from
other spots in the code base. Instead, we ensure that
`TopicView#include_suggested` and `TopicView#include_related` is set
correctly on the instance of `TopicView` which ensures that for the
given instance, `TopicView#suggested_topics` and
`TopicView#related_topics` will be a noop.

app/controllers/topics_controller.rb

@@ -1336,15 +1336,18 @@ class TopicsController < ApplicationController
       return
     end
 
+    if params[:replies_to_post_number] || params[:filter_upwards_post_id] ||
+         params[:filter_top_level_replies] || @topic_view.next_page.present?
+      @topic_view.include_suggested = false
+      @topic_view.include_related = false
+    end
+
     topic_view_serializer =
       TopicViewSerializer.new(
         @topic_view,
         scope: guardian,
         root: false,
         include_raw: !!params[:include_raw],
-        exclude_suggested_and_related:
-          !!params[:replies_to_post_number] || !!params[:filter_upwards_post_id] ||
-            !!params[:filter_top_level_replies],
       )
 
     respond_to do |format|