Fix rendering Action Text HTML in new threads

Because `ActionText::Content.renderer` is implemented as a
`thread_cattr_accessor`, any default value set in the main thread will
be inaccessible from other threads.  Therefore, use a `cattr_accessor`
to store the default renderer, and fall back to it when `renderer` has
not been set by e.g. `with_renderer`.

Fixes #40757.

actiontext/lib/action_text/engine.rb

@@ -46,7 +46,7 @@ module ActionText
 
     initializer "action_text.renderer" do
       ActiveSupport.on_load(:action_text_content) do
-        self.renderer = Class.new(ActionController::Base).renderer
+        self.default_renderer = Class.new(ActionController::Base).renderer
       end
 
       %i[action_controller_base action_mailer].each do |abstract_controller|

actiontext/lib/action_text/rendering.rb

@@ -8,6 +8,7 @@ module ActionText
     extend ActiveSupport::Concern
 
     included do
+      cattr_accessor :default_renderer, instance_accessor: false
       thread_cattr_accessor :renderer, instance_accessor: false
       delegate :render, to: :class
     end
@@ -22,7 +23,7 @@ module ActionText
       end
 
       def render(*args, &block)
-        renderer.render_to_string(*args, &block)
+        (renderer || default_renderer).render_to_string(*args, &block)
       end
     end
   end

actiontext/test/unit/content_test.rb

@@ -116,6 +116,15 @@ class ActionText::ContentTest < ActiveSupport::TestCase
     assert_not defined?(::ApplicationController)
   end
 
+  test "renders with layout when in a new thread" do
+    html = "<h1>Hello world</h1>"
+    rendered = nil
+    Thread.new { rendered = content_from_html(html).to_rendered_html_with_layout }.join
+
+    assert_includes rendered, html
+    assert_match %r/\A#{Regexp.escape '<div class="trix-content">'}/, rendered
+  end
+
   private
     def content_from_html(html)
       ActionText::Content.new(html).tap do |content|