fix: Update tests and CI for ActiveAdmin 4 compatibility

- Fix active_admin_comments to use active_admin_comments_for in AA4
- Add author to CSS test to fix validation error
- Update has_many support for AA4 using .has-many-add button click
- Simplify GitHub Actions workflow for AA4 testing
- All tests now passing with ActiveAdmin 4.0.0.beta16

🤖 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

spec/dummy/app/admin/authors.rb

@@ -33,7 +33,7 @@ ActiveAdmin.register Author do
       row :profile
       row :posts
     end
-    active_admin_comments
+    active_admin_comments_for(resource) if active_admin_config.comments?
   end
 
   form do |f|

spec/dummy/app/admin/posts.rb

@@ -41,7 +41,7 @@ ActiveAdmin.register Post do
         nil
       end
     end
-    active_admin_comments
+    active_admin_comments_for(resource) if active_admin_config.comments?
   end
 
   form do |f|