Record names were sent relative to the zone, derived from _sub_domain.
That has no valid form when the TXT name is the zone apex, which
happens with --domain-alias pointing at a dedicated zone:
- YC_Zone_ID: _sub_domain ends up empty and the API rejects the
request ("Length must be in the range of 1 to 254").
- YC_Folder_ID: _get_root returns the first label as _sub_domain, so
the record lands at <alias>.<alias>. and validation never succeeds.
The API accepts absolute names (trailing dot) everywhere a relative one
is accepted, so send $fulldomain, which is already lowercased and
dot-terminated. Ordinary _acme-challenge.<sub> names keep working.
Supersedes #6862.
