GeezerVison

Active Admin now builds its route helpers from the route set that ActiveAdmin.routes(self) is called in, so it works inside an engine's routes.draw. Until now, apps patched Helpers::Routes, BaseController and the Devise controllers to get there (#4294).

Decisions the diff doesn't show:

• ActionDispatch::Routing::Mapper has no public reader for its route set in Rails 7.2 to 8.1, so Application#routes reads @set, falling back to the host routes. A spec fails if that breaks.
• BaseController.inherited adds the engine's helpers to resource and page controllers. If controllers were loaded for another route set, ActiveAdmin.routes reloads them, so only one route set can host Active Admin. In an engine app on Rails 7.2 and 8.1, the engine's routes trigger the first load, so Active Admin and its before_load and after_load hooks still run once. A second load happens only if something loads Active Admin earlier, such as calling ActiveAdmin.routes in both the host and the engine.
• The Devise views use Devise's own path helpers, so they follow router_name. With router_name, Devise looks up root_path on that route set after sign-out, so Active Admin returns its own root there. An app's after_sign_out_path_for still wins. Apps with router_name: :main_app now sign out to the admin root, as apps without it already did.
• In engine mode, host-app helpers need main_app.. The docs say so.

Part of #9147. Request specs mount an engine at /backoffice and cover pages, sign-in and sign-out.