PHPUnit dependency mounts do not activate after database install

[chubes4]

Problem

The canonical PHPUnit recipe accepts inactive extra_plugins plus dependencyMounts, but current main does not activate or replay callbacks for those mounted plugins after the managed PHPUnit database is installed. The plugin source is mounted and Composer autoload setup sees it, yet activation state and plugins_loaded behavior are absent during tests.

This regresses the contract introduced by PR #1868.

Evidence

• Homeboy Extensions canary run against WP Codebox main: https://github.com/Extra-Chill/homeboy-extensions/actions/runs/29667332938
• Downstream repair PR that now reaches the real lifecycle: Load component during Codebox PHPUnit lifecycle Extra-Chill/homeboy-extensions#2302
• Original contract PR: Load PHPUnit dependencies after database install #1868

The run confirms both plugins are projected into setup:

{
  "plugins": [
    "test-db-activation-host/test-db-activation-host.php",
    "bench-db-activation-dep/bench-db-activation-dep.php"
  ]
}

PHPUnit then reports all three lifecycle assertions failing:

1. The dependency activation hook did not write its option.
2. The dependency activation hook did not complete get_users().
3. The dependency plugins_loaded callback did not fire.

Expected behavior

Plugins listed in dependencyMounts load and activate after wp-phpunit creates wptests_* tables, then callbacks registered for the dependency lifecycle execute exactly once before tests.

How to reproduce

1. Check out Homeboy Extensions PR #2302.
2. Check out WP Codebox main and build it.
3. Point HOMEBOY_WP_CODEBOX_BIN and HOMEBOY_WP_CODEBOX_CORE_MODULE at that build.
4. Run bash wordpress/scripts/test/playground-db-activation-smoke.sh.
5. Observe 3 tests and 3 failures.

Acceptance criteria

• Restore post-install activation for canonical PHPUnit dependency mounts.
• Replay newly registered dependency callbacks exactly once.
• Add real Playground integration coverage proving DB-touching activation and callback execution.
• Make the Homeboy Extensions #2302 canary pass.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI GPT-5.6 Sol with OpenCode
• Used for: Cross-repository lifecycle diagnosis, evidence analysis, and issue drafting.