fix(cron): prevent spin loop when job completes within scheduled second (#17821)

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

src/cron/service/jobs.ts

@@ -96,7 +96,18 @@ export function computeJobNextRunAtMs(job: CronJob, nowMs: number): number | und
             : null;
     return atMs !== null ? atMs : undefined;
   }
-  return computeNextRunAtMs(job.schedule, nowMs);
+  const next = computeNextRunAtMs(job.schedule, nowMs);
+  // Guard against the scheduler returning a time within the same second as
+  // nowMs.  When a cron job completes within the same wall-clock second it
+  // was scheduled for, some croner versions/timezone combinations may return
+  // the current second (or computeNextRunAtMs may return undefined, which
+  // triggers recomputation).  Advancing to the next second and retrying
+  // ensures we always land on the *next* occurrence.  (See #17821)
+  if (next === undefined && job.schedule.kind === "cron") {
+    const nextSecondMs = (Math.floor(nowMs / 1000) + 1) * 1000;
+    return computeNextRunAtMs(job.schedule, nextSecondMs);
+  }
+  return next;
 }
 
 /** Maximum consecutive schedule errors before auto-disabling a job. */