Add Ruby 4.0.0 to CI matrix

- Add Ruby 4.0.0 (released Dec 25, 2025) to the test matrix
- Update Ruby 3.4.4 to 3.4.8 (latest 3.4.x release)
- Note: Ruby 3.5 stable was skipped in favor of 4.0

Author: claude

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ jobs:
     name: 'Test Suite'
     strategy:
       matrix:
-        ruby: [3.2.8, 3.3.8, 3.4.4]
+        ruby: [3.2.8, 3.3.8, 3.4.8, 4.0.0]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2