Deduplicate PHPUnit command PHP fragments

[chubes4]

Summary

• Deduplicates shared embedded PHP fragments used by plugin and core PHPUnit command handlers.
• Keeps plugin/core-specific bootstrapping separate while sharing config discovery, test discovery, changed-test filtering, and PHPUnit arg normalization generation.
• Preserves existing generated PHP function names so command smoke contracts remain stable.

Fixes #770

Testing

• npm run build
• npm run core-phpunit-command-smoke
• npm run phpunit-diagnostic-artifact-smoke
• npm run project-phpunit-bootstrap-smoke
• npm run wordpress-plugin-smoke
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the focused refactor and ran the verification checks; Chris remains responsible for review and merge.