Enable frozen strings in remaining lib/banzai/filter/*.rb files

This also fixes a bug with the ImageLazyLoadFilter not doing the right
thing when an existing class attribute is present.

Part of #47424

changelogs/unreleased/sh-enable-frozen-literals-banzi-filters.yml

+---
+title: Enable frozen strings in remaining lib/banzai/filter/*.rb files
+merge_request: 20777
+author:
+type: performance

lib/banzai/filter/image_lazy_load_filter.rb

+# frozen_string_literal: true
+
 module Banzai
   module Filter
     # HTML filter that moves the value of image `src` attributes to `data-src`
@@ -5,7 +7,7 @@ module Banzai
     class ImageLazyLoadFilter < HTML::Pipeline::Filter
       def call
         doc.xpath('descendant-or-self::img').each do |img|
-          img['class'] ||= '' << 'lazy'
+          img.add_class('lazy')
           img['data-src'] = img['src']
           img['src'] = LazyImageTagHelper.placeholder_image
         end

lib/banzai/filter/syntax_highlight_filter.rb

+# frozen_string_literal: true
+
 require 'rouge/plugins/common_mark'
 require 'rouge/plugins/redcarpet'
 
@@ -15,7 +17,7 @@ module Banzai
       end
 
       def highlight_node(node)
-        css_classes = 'code highlight js-syntax-highlight'
+        css_classes = +'code highlight js-syntax-highlight'
         lang = node.attr('lang')
         retried = false
 

lib/banzai/filter/table_of_contents_filter.rb

+# frozen_string_literal: true
+
 module Banzai
   module Filter
     # HTML filter that adds an anchor child element to all Headers in a
@@ -19,7 +21,7 @@ module Banzai
       def call
         return doc if context[:no_header_anchors]
 
-        result[:toc] = ""
+        result[:toc] = +""
 
         headers = Hash.new(0)
         header_root = current_header = HeaderNode.new

spec/lib/banzai/filter/image_lazy_load_filter_spec.rb

@@ -7,6 +7,20 @@ describe Banzai::Filter::ImageLazyLoadFilter do
     %(<img src="#{path}" />)
   end
 
+  def image_with_class(path, class_attr = nil)
+    %(<img src="#{path}" class="#{class_attr}"/>)
+  end
+
+  it 'adds a class attribute' do
+    doc = filter(image('/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg'))
+    expect(doc.at_css('img')['class']).to eq 'lazy'
+  end
+
+  it 'appends to the current class attribute' do
+    doc = filter(image_with_class('/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg', 'test'))
+    expect(doc.at_css('img')['class']).to eq 'test lazy'
+  end
+
   it 'transforms the image src to a data-src' do
     doc = filter(image('/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg'))
     expect(doc.at_css('img')['data-src']).to eq '/uploads/e90decf88d8f96fe9e1389afc2e4a91f/test.jpg'