Fix CI: downgrade doctrine/instantiator for PHP 8.3 and use phpunit directly

- Downgrade doctrine/instantiator from 2.1.0 to 2.0.0 in lock file
  (2.1.0 requires PHP 8.4, but Docker image uses PHP 8.3)
- Run phpunit directly in CI instead of composer test since the
  final Docker image doesn't include composer

https://claude.ai/code/session_01UCN4sKD3Zkdbimn3QgTHSB

Author: claude

.github/workflows/tests.yml

@@ -16,4 +16,4 @@ jobs:
           docker compose up -d --wait
 
       - name: Run Tests
-        run: docker compose exec usage composer test
+        run: docker compose exec usage php vendor/bin/phpunit