[Admin] Use Rails.application.mounted_helpers in base component

Rather than implementing routing proxies ourselves, we can use what
Rails offers.

Fixes solidusio#6038

admin/app/components/solidus_admin/base_component.rb

@@ -37,23 +37,5 @@ def self.stimulus_id
     end
 
     delegate :stimulus_id, to: :class
-
-    class << self
-      private
-
-      def engines_with_routes
-        Rails::Engine.subclasses.map(&:instance).reject do |engine|
-          engine.routes.empty?
-        end
-      end
-    end
-
-    # For each engine with routes, define a method that returns the routes proxy.
-    # This allows us to use the routes in the context of a component class.
-    engines_with_routes.each do |engine|
-      define_method(engine.engine_name) do
-        engine.routes.url_helpers
-      end
-    end
   end
 end

admin/lib/solidus_admin/engine.rb

@@ -63,5 +63,11 @@ class Engine < ::Rails::Engine
         SolidusAdmin::Engine.root.join("app/components"),
       ]
     end
+
+    initializer "solidus_admin.routing_proxies" do |app|
+      ActiveSupport.on_load(:after_routes_loaded) do
+        SolidusAdmin::BaseComponent.include app.routes.mounted_helpers
+      end
+    end
   end
 end

admin/spec/components/solidus_admin/base_component_spec.rb

@@ -20,23 +20,19 @@ def call
 
   describe "#spree" do
     it "gives access to spree routing helpers" do
-      without_partial_double_verification do
-        allow(Spree::Core::Engine.routes.url_helpers).to receive(:foo_path).and_return("/foo/bar")
-      end
-      component = described_class.new
-
-      expect(component.spree.foo_path).to eq("/foo/bar")
+      expect(described_class.new).to respond_to(:spree)
     end
   end
 
   describe "#solidus_admin" do
     it "gives access to solidus_admin routing helpers" do
-      without_partial_double_verification do
-        allow(SolidusAdmin::Engine.routes.url_helpers).to receive(:foo_path).and_return("/foo/bar")
-      end
-      component = described_class.new
+      expect(described_class.new).to respond_to(:solidus_admin)
+    end
+  end
 
-      expect(component.solidus_admin.foo_path).to eq("/foo/bar")
+  describe "#main_app" do
+    it "gives access to main_app routing helpers" do
+      expect(described_class.new).to respond_to(:main_app)
     end
   end