(a) Probe-failure detection: now treats empty OR unparseable JSON as
probe-failed, not never-run. This prevents 'command not found'
outputs from being rendered as service verdicts.
(b) pending-bytes: now extracted from JSON and reported in stale verdict.
(c) Null endtime: use .get() with explicit None check, not 0 fallback.
null values now correctly trigger never-run verdict instead of
arithmetic crash (set -u).
(d) Tests: Added 14-assertion stub-driven suite covering: healthy,
stale (>48h), probe-failed (empty and unparseable), null endtime,
datastore absent. Each test stubs ssh/curl to verify exact
behavior against the pre-fix head.
Branch: fix/pbs-gc-liveness-signal-20260919
