Add two arbiter regression tests. A worker exit is still reaped normally (tmp
closed, child_exit called) while a companion manager pid is registered, so the
companion reap branch does not swallow worker exits. And an HTTP worker is
still spawned and recorded as before when companions are configured, so the
companion config never touches the worker path.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>