fix(cron): migrate legacy schedule cron fields on load (#28889)

Backfill legacy jobs that still use schedule.cron and jobId so upgraded instances keep firing existing cron schedules instead of failing silently.

Closes #28861

src/cron/schedule.test.ts

@@ -25,6 +25,19 @@ describe("cron schedule", () => {
     ).toThrow("invalid cron schedule: expr is required");
   });
 
+  it("supports legacy cron field when expr is missing", () => {
+    const nowMs = Date.parse("2025-12-13T00:00:00.000Z");
+    const next = computeNextRunAtMs(
+      {
+        kind: "cron",
+        cron: "0 9 * * 3",
+        tz: "America/Los_Angeles",
+      } as unknown as { kind: "cron"; expr: string; tz?: string },
+      nowMs,
+    );
+    expect(next).toBe(Date.parse("2025-12-17T17:00:00.000Z"));
+  });
+
   it("computes next run for every schedule", () => {
     const anchor = Date.parse("2025-12-13T00:00:00.000Z");
     const now = anchor + 10_000;