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/timer.ts

@@ -15,6 +15,15 @@ import { ensureLoaded, persist } from "./store.js";
 
 const MAX_TIMER_DELAY_MS = 60_000;
 
+/**
+ * Minimum gap between consecutive fires of the same cron job.  This is a
+ * safety net that prevents spin-loops when `computeJobNextRunAtMs` returns
+ * a value within the same second as the just-completed run.  The guard
+ * is intentionally generous (2 s) so it never masks a legitimate schedule
+ * but always breaks an infinite re-trigger cycle.  (See #17821)
+ */
+const MIN_REFIRE_GAP_MS = 2_000;
+
 /**
  * Maximum wall-clock time for a single job execution. Acts as a safety net
  * on top of the per-provider / per-agent timeouts to prevent one stuck job
@@ -107,7 +116,18 @@ function applyJobResult(
         "cron: applying error backoff",
       );
     } else if (job.enabled) {
-      job.state.nextRunAtMs = computeJobNextRunAtMs(job, result.endedAt);
+      const naturalNext = computeJobNextRunAtMs(job, result.endedAt);
+      if (job.schedule.kind === "cron") {
+        // Safety net: ensure the next fire is at least MIN_REFIRE_GAP_MS
+        // after the current run ended.  Prevents spin-loops when the
+        // schedule computation lands in the same second due to
+        // timezone/croner edge cases (see #17821).
+        const minNext = result.endedAt + MIN_REFIRE_GAP_MS;
+        job.state.nextRunAtMs =
+          naturalNext !== undefined ? Math.max(naturalNext, minNext) : minNext;
+      } else {
+        job.state.nextRunAtMs = naturalNext;
+      }
     } else {
       job.state.nextRunAtMs = undefined;
     }