Fix N+1 problem in CustomEmojiFilter

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/271242

**Problem**

I discovered repetitive `SELECT 1 AS one FROM "custom_emoji" WHERE
"custom_emoji"."namespace_id" = <id> LIMIT 1` queries generated by
CustomEmojiFilter.

**Solution**

- Update test to catch N+1 queries
- Add memoization to prevent them

changelogs/unreleased/271242_fix_n_1_in_custom_emoji_filter.yml

+---
+title: Fix N+1 problem in CustomEmojiFilter
+merge_request: 60910
+author:
+type: performance

lib/banzai/filter/custom_emoji_filter.rb

@@ -3,6 +3,8 @@
 module Banzai
   module Filter
     class CustomEmojiFilter < HTML::Pipeline::Filter
+      include Gitlab::Utils::StrongMemoize
+
       IGNORED_ANCESTOR_TAGS = %w(pre code tt).to_set
 
       def call
@@ -14,7 +16,7 @@ module Banzai
 
           next if has_ancestor?(node, IGNORED_ANCESTOR_TAGS)
           next unless content.include?(':')
-          next unless namespace && namespace.custom_emoji.any?
+          next unless has_custom_emoji?
 
           html = custom_emoji_name_element_filter(content)
 
@@ -46,6 +48,12 @@ module Banzai
 
       private
 
+      def has_custom_emoji?
+        strong_memoize(:has_custom_emoji) do
+          namespace&.custom_emoji&.any?
+        end
+      end
+
       def namespace
         context[:project].namespace.root_ancestor
       end

spec/lib/banzai/filter/custom_emoji_filter_spec.rb

@@ -53,7 +53,7 @@ RSpec.describe Banzai::Filter::CustomEmojiFilter do
     end
 
     expect do
-      filter('<p>:tanuki: :party-parrot:</p>')
+      filter('<p>:tanuki:</p> <p>:party-parrot:</p>')
     end.not_to exceed_all_query_limit(control_count.count)
   end
 end