Fail closed on PHPUnit discovery errors

[chubes4]

Summary

• allow empty PHPUnit success only for a valid, non-empty changed-test scope that matches no tests
• fail on missing test roots, missing or malformed XML configuration, and generic zero-test discovery
• preserve implicit phpunit.xml.dist to phpunit.xml fallback while keeping explicit configuration fail-closed before bootstrap execution

Verification

• npm run build
• 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 #1933. Addresses the empty-suite regression documented in #1916.