FIX: Ensure ordering is enforced in PostsController#replies (#31826)

This was picked up by the `PostsController#replies supports pagination`
requests spec as it was flaky from time to time.

app/controllers/posts_controller.rb

@@ -451,6 +451,7 @@ class PostsController < ApplicationController
         .replies
         .secured(guardian)
         .where(post_number: after + 1..)
+        .order(:post_number)
         .limit(MAX_POST_REPLIES)
         .pluck(:id)
 

spec/requests/posts_controller_spec.rb

@@ -211,25 +211,32 @@ RSpec.describe PostsController do
 
     it "supports pagination" do
       parent = Fabricate(:post)
+
+      child_posts = []
+
       30.times do
         reply = Fabricate(:post, topic: parent.topic, reply_to_post_number: parent.post_number)
         PostReply.create!(post: parent, reply:)
+        child_posts << reply
       end
 
       get "/posts/#{parent.id}/replies.json", params: { after: parent.post_number }
       expect(response.status).to eq(200)
       replies = response.parsed_body
-      expect(replies.size).to eq(20)
+
+      expect(replies.map { |reply| reply["id"] }).to eq(child_posts[0..19].map(&:id))
 
       after = replies.last["post_number"]
 
       get "/posts/#{parent.id}/replies.json", params: { after: }
       expect(response.status).to eq(200)
       replies = response.parsed_body
-      expect(replies.size).to eq(10)
+
+      expect(replies.map { |reply| reply["id"] }).to eq(child_posts[20..-1].map(&:id))
       expect(replies[0][:post_number]).to eq(after + 1)
 
       get "/posts/#{parent.id}/replies.json", params: { after: 999_999 }
+
       expect(response.status).to eq(200)
       expect(response.parsed_body.size).to eq(0)
     end