Load PHPUnit dependencies after database install

[chubes4]

Summary

• load and activate mounted PHPUnit dependency plugins after the managed test database is installed
• replay only dependency plugins_loaded callbacks registered during post-install loading
• document the canonical inactive extra_plugins plus dependencyMounts contract
• add real Playground coverage for DB-touching activation and one-time callback execution

Why

WP Codebox previously forced validation dependencies into two broken choices: activate during recipe setup before wptests_* tables exist, or mount inactive and never load them during managed PHPUnit execution.

This surfaced after Extra-Chill/homeboy-extensions#2294 delegated its test runner to the public WP Codebox recipe contract.

Failure evidence: https://github.com/Extra-Chill/homeboy-extensions/actions/runs/29646694037/job/88086037491
Downstream repair: Extra-Chill/homeboy-extensions#2295

How to test

1. Run npm install from a fresh checkout.
2. Run npm run build.
3. Run npx tsx tests/phpunit-project-autoload.test.ts.
4. Run npm run test:playground-phpunit-readonly-cache-integration.
5. In Homeboy Extensions PR #2295, run bash wordpress/scripts/test/playground-db-activation-smoke.sh against this branch.

Compatibility

The canonical caller contract is inputs.extra_plugins with dependency entries set to activate: false, plus their sandbox plugin directories in dependencyMounts. Dependencies now activate after managed PHPUnit installation and before tests. Newly registered dependency plugins_loaded callbacks run exactly once. No legacy lifecycle or Homeboy-specific behavior is introduced.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-sol via OpenCode
• Used for: Reproduced the downstream canary failure, implemented the generic PHPUnit lifecycle repair, added integration coverage, and prepared the PR under Chris Huber's direction.

[chubes4]

Merge dependency: #1869 fixes the pre-existing main production-boundary failure inherited by this PR. Recommended order: #1869, then #1868, then Extra-Chill/homeboy-extensions#2295.