chore: add Ruby 4.0 to CI matrix

Test against all supported Ruby versions: 3.2, 3.3, 3.4, 4.0.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: markhallen

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby-version: ['3.2', '3.3', '3.4']
+        ruby-version: ['3.2', '3.3', '3.4', '4.0']
 
     steps:
     - uses: actions/checkout@v4