Limit the length of generated anchor URL for markdown headers

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/204808

**Problem**

There is no limitation for generated header anchors. It might cause
performance problems for long headers processing.

**Solution**

Limit anchor link length with 255 characters.

Changelog: changed

lib/banzai/filter/table_of_contents_filter.rb

@@ -32,7 +32,7 @@ module Banzai
 
         doc.xpath(XPATH).each do |node|
           if header_content = node.children.first
-            id = string_to_anchor(node.text)
+            id = string_to_anchor(node.text[0...255])
 
             uniq = headers[id] > 0 ? "-#{headers[id]}" : ''
             headers[id] += 1

spec/lib/banzai/filter/table_of_contents_filter_spec.rb

@@ -91,6 +91,12 @@ RSpec.describe Banzai::Filter::TableOfContentsFilter do
         # ExternalLinkFilter (see https://gitlab.com/gitlab-org/gitlab/issues/26210)
         expect(doc.css('h1 a').first.attr('href')).to eq "##{CGI.escape('한글')}"
       end
+
+      it 'limits header href length with 255 characters' do
+        doc = filter(header(1, 'a' * 500))
+
+        expect(doc.css('h1 a').first.attr('href')).to eq "##{'a' * 255}"
+      end
     end
   end