Vitest TEST_TMP_DIR and SENTRY_CONFIG_DIR isolation

Trap: deleting a worker-wide TEST_TMP_DIR or SENTRY_CONFIG_DIR looks like clean isolation because each test uses child paths, but Vitest can reload setup within one pool while other files still use that root, causing SQLite disk I/O error and mkdtemp ENOENT. Fix: namespace roots by VITEST_POOL_ID; preload removes only its process child; helpers recreate the parent before mkdtemp; database tests use useTestConfigDir() and always restore the saved environment value, never delete it.