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/service.store-migration.test.ts

@@ -148,4 +148,59 @@ describe("CronService store migrations", () => {
     cron.stop();
     await store.cleanup();
   });
+
+  it("migrates legacy cron fields (jobId + schedule.cron) and defaults wakeMode", async () => {
+    const store = await makeStorePath();
+    await fs.mkdir(path.dirname(store.storePath), { recursive: true });
+    await fs.writeFile(
+      store.storePath,
+      JSON.stringify(
+        {
+          version: 1,
+          jobs: [
+            {
+              jobId: "legacy-cron-field-job",
+              name: "legacy cron field",
+              enabled: true,
+              createdAtMs: Date.parse("2026-02-01T12:00:00.000Z"),
+              updatedAtMs: Date.parse("2026-02-05T12:00:00.000Z"),
+              schedule: { kind: "cron", cron: "*/5 * * * *", tz: "UTC" },
+              payload: { kind: "systemEvent", text: "tick" },
+              state: {},
+            },
+          ],
+        },
+        null,
+        2,
+      ),
+      "utf-8",
+    );
+
+    const cron = await createStartedCron(store.storePath).start();
+    const jobs = await cron.list({ includeDisabled: true });
+    const job = jobs.find((entry) => entry.id === "legacy-cron-field-job");
+    expect(job).toBeDefined();
+    expect(job?.wakeMode).toBe("now");
+    expect(job?.schedule.kind).toBe("cron");
+    if (job?.schedule.kind === "cron") {
+      expect(job.schedule.expr).toBe("*/5 * * * *");
+    }
+
+    const persisted = JSON.parse(await fs.readFile(store.storePath, "utf-8")) as {
+      jobs: Array<Record<string, unknown>>;
+    };
+    const persistedJob = persisted.jobs.find((entry) => entry.id === "legacy-cron-field-job");
+    expect(persistedJob).toBeDefined();
+    expect(persistedJob?.jobId).toBeUndefined();
+    expect(persistedJob?.wakeMode).toBe("now");
+    const persistedSchedule =
+      persistedJob?.schedule && typeof persistedJob.schedule === "object"
+        ? (persistedJob.schedule as Record<string, unknown>)
+        : null;
+    expect(persistedSchedule?.cron).toBeUndefined();
+    expect(persistedSchedule?.expr).toBe("*/5 * * * *");
+
+    cron.stop();
+    await store.cleanup();
+  });
 });