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/system/css_loading_spec.rb

@@ -69,7 +69,8 @@ RSpec.describe 'CSS Loading', type: :system do
 
     it 'applies Quill styles correctly when editor is present' do
       # Create a test model with Quill editor
-      post = Post.create!(title: 'Test Post', description: '<p>Test content</p>')
+      author = Author.create!(email: 'test@example.com', name: 'Test Author', age: 30)
+      post = Post.create!(author: author, title: 'Test Post', description: '<p>Test content</p>')
       visit "/admin/posts/#{post.id}/edit"
 
       # Wait for editor to load