DEV: Ensure that `BlockRequestsMiddleware` cookie is always set (#25826)

Why this change?

This reverts 725561cf4b as it did not
address the root cause of the problem even though it fixed the failing tests we were seeing 
when running `bundle exec rspec --tag ~type:multisite --order random:776 spec/system/admin_customize_form_templates_spec.rb spec/system/admin_sidebar_navigation_spec.rb spec/system/admin_site_setting_search_spec.rb spec/system/composer/dont_feed_the_trolls_popup_spec.rb spec/system/composer/review_media_unless_trust_level_spec.rb spec/system/create_account_spec.rb spec/system/editing_sidebar_tags_navigation_spec.rb spec/system/email_change_spec.rb spec/system/emojis/emoji_deny_list_spec.rb spec/system/group_activity_spec.rb spec/system/hashtag_autocomplete_spec.rb spec/system/network_disconnected_spec.rb spec/system/post_menu_spec.rb spec/system/post_small_action_spec.rb spec/system/tags_intersection_spec.rb spec/system/topic_list_focus_spec.rb spec/system/topic_page_spec.rb spec/system/user_page/user_profile_info_panel_spec.rb spec/system/viewing_group_members_spec.rb spec/system/viewing_navigation_menu_preferences_spec.rb`.

The root cause here is that `before_action`s added to a controller is
order dependent. As such, some requests were not setting the cookie
because the `before_action` callback was not even hit as a prior
`before_action` callbacks has raised an error such as the `check_xhr`
`before_action` callback.

To resolve the problem, we need to add the `prepend: true` option in
our monkey patch of `ApplicationController` to ensure that the
`before_action` callback which we have added is always run first.

This change also makes a couple of changes:

1. Improve the response body when a request is blocked by the `BlockRequestsMiddleware` middleware
   so that it makes debugging easier.

2. Only set the cookies for non-xhr HTML format requests. Setting it for
   other formats is kind of pointless.

config/initializers/200-first_middlewares.rb

@@ -58,12 +58,18 @@ if Rails.env.test?
            (
              @@block_requests ||
                (
-                 request.cookies[RSPEC_CURRENT_EXAMPLE_COOKIE_STRING].present? &&
+                 self.class.current_example_location.present? &&
                    self.class.current_example_location !=
                      request.cookies[RSPEC_CURRENT_EXAMPLE_COOKIE_STRING]
                )
            )
-        [503, { "Content-Type" => "text/plain" }, ["Blocked by BlockRequestsMiddleware"]]
+        [
+          503,
+          { "Content-Type" => "text/plain" },
+          [
+            "Blocked by BlockRequestsMiddleware for requests initiated by #{request.cookies[RSPEC_CURRENT_EXAMPLE_COOKIE_STRING]} when running #{self.class.current_example_location}",
+          ],
+        ]
       else
         @app.call(env)
       end

spec/rails_helper.rb

@@ -593,8 +593,9 @@ RSpec.configure do |config|
         ActiveRecord::Base.connection.schema_cache.add(table)
       end
 
-      ApplicationController.before_action do
-        if BlockRequestsMiddleware.current_example_location
+      ApplicationController.before_action(prepend: true) do
+        if BlockRequestsMiddleware.current_example_location && !request.xhr? &&
+             request.format == "html"
           cookies[
             BlockRequestsMiddleware::RSPEC_CURRENT_EXAMPLE_COOKIE_STRING
           ] = BlockRequestsMiddleware.current_example_location