Make Redcarpet Markdown renderer thread-safe

The Redcarpet library is not thread-safe as described in
https://github.com/vmg/redcarpet/issues/570. Since we instantiate
the Redcarpet renderer in a class variable, multiple Sidekiq threads
can access the work buffer and corrupt the state. We work around
this issue by memoizing the renderer on a thread basis.

Closes #36637

changelogs/unreleased/sh-thread-safe-markdown.yml

+---
+title: Make Redcarpet Markdown renderer thread-safe
+merge_request:
+author:
+type: fixed

lib/banzai/filter/markdown_filter.rb

 module Banzai
   module Filter
     class MarkdownFilter < HTML::Pipeline::TextFilter
+      # https://github.com/vmg/redcarpet#and-its-like-really-simple-to-use
+      REDCARPET_OPTIONS = {
+        fenced_code_blocks:  true,
+        footnotes:           true,
+        lax_spacing:         true,
+        no_intra_emphasis:   true,
+        space_after_headers: true,
+        strikethrough:       true,
+        superscript:         true,
+        tables:              true
+      }.freeze
+
       def initialize(text, context = nil, result = nil)
         super text, context, result
         @text = @text.delete "\r"
@@ -13,27 +25,11 @@ module Banzai
       end
 
       def self.renderer
-        @renderer ||= begin
+        Thread.current[:banzai_markdown_renderer] ||= begin
           renderer = Banzai::Renderer::HTML.new
-          Redcarpet::Markdown.new(renderer, redcarpet_options)
-        end
+          Redcarpet::Markdown.new(renderer, REDCARPET_OPTIONS)
         end
-
-      def self.redcarpet_options
-        # https://github.com/vmg/redcarpet#and-its-like-really-simple-to-use
-        @redcarpet_options ||= {
-          fenced_code_blocks:  true,
-          footnotes:           true,
-          lax_spacing:         true,
-          no_intra_emphasis:   true,
-          space_after_headers: true,
-          strikethrough:       true,
-          superscript:         true,
-          tables:              true
-        }.freeze
       end
-
-      private_class_method :redcarpet_options
     end
   end
 end