A missing zone raised before any tunnel was checked, so it blocked teardown
even with no tunnels left. The sweep now skips the zone lookup when nothing
remains, and still removes tunnels when the zone is gone. A 404 on delete counts
as swept; any other failure, such as a 429, still fails so the next run retries.
Covered by tests that drive the sweep through a stubbed Cloudflare client.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>