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>

.github/workflows/specs.yml

@@ -102,12 +102,10 @@ jobs:
         run: |
           gem install bundler
           bundle config set --local without 'development'
-          bundle add activeadmin --version '~> 4.0.0.beta' --skip-install
-          bundle add importmap-rails --skip-install
-          bundle add propshaft --skip-install if [ "${{ matrix.rails }}" = "7.2" ]
           bundle install --jobs 4 --retry 3
         env:
           RAILS_VERSION: ${{ matrix.rails }}
+          ACTIVEADMIN_VERSION: ''
 
       - name: Set up Node.js
         uses: actions/setup-node@v4
@@ -118,7 +116,6 @@ jobs:
         run: |
           cd spec/dummy
           npm install
-          npm install @activeadmin/activeadmin@^4.0.0-beta
 
       - name: Build assets
         run: |