test(labels): exercise the priority-label path's six degraded branches (#9866) (#9867)

Every I/O call in maybeHandlePriorityLabelEligibility is wrapped in a .catch that
degrades rather than failing the webhook delivery -- which is what makes a
label-mutating rule safe to run on a webhook at all, since an escaping throw would
take every handler after it on the same event down with it.

None of the six was exercised: the existing fail-open case asserts a 404 RESPONSE,
which takes the ordinary return path, not the catch. These make each call actually
throw and assert the degraded outcome.

Author: JSONbored