When a cron job fires and completes within the same wall-clock second it
was scheduled for, the next-run computation could return undefined or the
same second, causing the scheduler to re-trigger the job hundreds of
times in a tight loop.
Two-layer fix:
1. computeJobNextRunAtMs: When computeNextRunAtMs returns undefined for a
cron-kind schedule (edge case where floored nowSecondMs matches the
schedule), retry with the ceiling (next second) as reference time.
This ensures we always get the next valid occurrence.
2. applyJobResult: Add MIN_REFIRE_GAP_MS (2s) safety net for cron-kind
jobs. After a successful run, nextRunAtMs is guaranteed to be at
least 2s in the future. This breaks any remaining spin-loop edge
cases without affecting normal daily/hourly schedules (where the
natural next run is hours/days away).
Fixes#17821
8af4712c40