Add a CI smoke test for the self-locating bootstrap in an isolated child process

Start with src/Unit/bootstrap.php and src/Integration/bootstrap.php, then inspect the existing GitHub Actions workflows. Run a smoke test without defining the WPMEDIA_PHPUNIT_* or WP_TESTS_DIR constants, requiring a bootstrap and checking both derived constants. Done means CI runs it on every push, a second bootstrap invocation is safe, and regressions fail loudly.

🤖 AI-generated — created by an automated pipeline. Review before acting on this.

Context
PR #55 (issue #52) made src/Unit/bootstrap.php and src/Integration/bootstrap.php self-locating: when WPMEDIA_PHPUNIT_ROOT_DIR is not already defined, each bootstrap now require_onces src/BootstrapManager.php and calls BootstrapManager::setupConstants('unit'|'integration') to derive the two WPMEDIA_PHPUNIT_* constants. This is the exact code path that runs when PHPUnit re-executes the bootstrap standalone in an isolated @runInSeparateProcess child.

However, the package's own test suite cannot exercise this new branch because its bootstraps go through Tests/{Unit,Integration}/init-tests.php, which pre-defines the constants first. This means the self-derivation branch is validated today only by manual smoke testing, not by CI. A regression (e.g., a wrong relative path in the require_once, or a change to getRootDir()/setupConstants()) could silently break isolated-child consumers without any CI signal.

Dependencies

• Issue #52 / PR #55 (the self-locating changes themselves) — should already be merged

What needs to be done
Add a small CI smoke test script (e.g., a standalone PHP script or harness in a dedicated .github/workflows/ step) that:

1. Does NOT pre-define WPMEDIA_PHPUNIT_ROOT_DIR, WPMEDIA_PHPUNIT_ROOT_TEST_DIR, or WP_TESTS_DIR
2. Requires the self-locating guard logic from src/Unit/bootstrap.php (or src/Integration/bootstrap.php)
3. Validates the self-derivation behavior
4. Consider also testing a synthetic vendor/wp-media/phpunit consumer layout to mirror the real isolated-child scenario

Acceptance Criteria

• CI step (or new workflow) runs the smoke test on every push
• Smoke test confirms both WPMEDIA_PHPUNIT_ROOT_DIR and WPMEDIA_PHPUNIT_ROOT_TEST_DIR are defined after bootstrap require
• No fatal "Undefined constant" errors occur during the smoke test
• A second invocation of the bootstrap short-circuits (no double-define errors under convertWarningsToExceptions)
• CI fails loudly if the self-locating behavior regresses

Additional information
This hardens the #52 behavior so it cannot silently regress. It is not a blocker but strongly improves confidence in the isolated-process bootstrap contract for downstream consumers.

Contributes to #30. Follow-up to #52 / PR #55.