Restore configless managed PHPUnit discovery

[chubes4]

Summary

• let implicit plugin PHPUnit profiles use managed discovery defaults when neither default XML file exists
• keep explicit plugin and WordPress core configuration failures fail-closed
• execute generated PHP discovery against a real configless WP_UnitTestCase fixture

Root cause

PR #1940 required the implicit phpunit.xml.dist path to be readable, but the recipe builder only supplies that path and never materializes the file. Configless plugin suites therefore failed before discovery.

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
• npm run test:playground-phpunit-readonly-cache-integration
• git diff --check

Closes #1943.
Downstream proof tracker: Extra-Chill/homeboy-extensions#2343

AI assistance

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