Fix private PHPUnit TestSuite constructor compatibility

[chubes4]

Summary

• construct generated plugin and WordPress core PHPUnit suites through TestSuite::empty() when that public factory exists
• retain the public constructor fallback for PHPUnit 9 and pass the already-created ReflectionClass to addTestSuite() across API generations
• execute both generated harness tails against private-constructor and legacy public-constructor PHPUnit doubles, proving discovered test files reach run_tests

Root cause

Both generated harness paths in packages/runtime-playground/src/phpunit-command-handlers.ts directly called new PHPUnit\\Framework\\TestSuite(...) during load_tests. PHPUnit 10 and newer expose a private constructor and require the public TestSuite::empty() factory, so the generated PHP terminated before any discovered component test executed. The same paths also passed a class-name string to addTestSuite(), while newer PHPUnit versions require a ReflectionClass.

The suite creation is now guarded by method_exists(TestSuite::class, 'empty'): PHPUnit 10+ uses the public factory, while PHPUnit 9, which has no empty() method, retains its public constructor. Passing ReflectionClass works in PHPUnit 9 and is required by newer versions. Factory/construction errors remain classified as load_tests infrastructure failures rather than workload assertion failures.

Downstream tracking: Extra-Chill/homeboy-extensions#2258.

Test evidence

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

The focused regression executes plugin and core generated harness tails in both compatibility modes and asserts STAGE_OK:load_tests, STAGE_BEGIN:run_tests, STAGE_OK:run_tests, no infrastructure failure marker, and an execution marker written by the discovered test.

Closes #1926