Fall back to Devise.available_router_name when mapping has none

[Fivell]

Summary

Follow-up to the previous engine-mounted-Devise fix. Hosts that pin URL helpers via Devise.router_name = :engine in the devise initializer (the global setting) — but don't also pass router_name: to devise_for — still hit NoMethodError because Devise.mappings[scope].router_name is nil for them and the previous patch fell through to self.

Why

Devise's own URL-helper dispatcher (devise/controllers/url_helpers.rb#generate_helpers!) uses both:

router_name = Devise.mappings[scope].router_name
context = router_name ? send(router_name) : _devise_route_context

def _devise_route_context
  @_devise_route_context ||= send(Devise.available_router_name)
end

The previous gem fix only honored the per-mapping value. This commit adds the Devise.available_router_name fallback so the dispatcher matches Devise's own behavior:

def after_omniauth_failure_path_for(scope)
  router_name = ::Devise.mappings[scope].router_name ||
                ::Devise.available_router_name
  send(router_name).public_send(:"new_#{scope}_session_path")
end

Devise.available_router_name defaults to :main_app, so the default-app case still resolves through the main_app proxy — no special branching needed.

Test coverage

spec/dummy_engine/lib/admin_panel/config/routes.rb now uses the global-only pattern (devise_for :admin_users, ActiveAdmin::Devise.config with no router_name:), so the existing failure-path scenario in spec/engine/features/engine_mounted_login_spec.rb exercises this code path. It would fail with the previous patch and passes with this one.

Test plan

• rake spec:all is green — covers default, engine (global-only), and isolated-engine setups.