Add PHPUnit config to readonly cache fixture

[chubes4]

Summary

• add a minimal valid phpunit.xml.dist to the readonly-cache integration fixture
• include the config in the readonly source integrity digest
• preserve fail-closed runtime behavior introduced by Fail closed on PHPUnit discovery errors #1940

Verification

• npm run build
• npm run test:playground-phpunit-readonly-cache-integration
• npx tsx tests/phpunit-project-autoload.test.ts
• npm run test:phpunit-runtime-failure-diagnostics
• npm run test:playground-phpunit-bootstrap-failure-integration
• git diff --check

Follow-up to #1940 after the contract suite exposed the stale fixture assumption.