fix(cron): use separate schema for failureDestination and fix type cast

- Create CronFailureDestinationSchema excluding after/cooldownMs fields
- Fix type cast in sendFailureNotificationAnnounce to use CronMessageChannel

src/cron/delivery.ts

@@ -215,7 +215,7 @@ export async function sendFailureNotificationAnnounce(
 ): Promise<void> {
   const cfg = loadConfig();
   const resolvedTarget = await resolveDeliveryTarget(cfg, agentId, {
-    channel: target.channel as "last" | undefined,
+    channel: target.channel as CronMessageChannel | undefined,
     to: target.to,
     accountId: target.accountId,
   });

src/gateway/protocol/schema/cron.ts

@@ -150,11 +150,21 @@ export const CronFailureAlertSchema = Type.Object(
   { additionalProperties: false },
 );
 
+export const CronFailureDestinationSchema = Type.Object(
+  {
+    channel: Type.Optional(Type.Union([Type.Literal("last"), NonEmptyString])),
+    to: Type.Optional(Type.String()),
+    accountId: Type.Optional(NonEmptyString),
+    mode: Type.Optional(Type.Union([Type.Literal("announce"), Type.Literal("webhook")])),
+  },
+  { additionalProperties: false },
+);
+
 const CronDeliverySharedProperties = {
   channel: Type.Optional(Type.Union([Type.Literal("last"), NonEmptyString])),
   accountId: Type.Optional(NonEmptyString),
   bestEffort: Type.Optional(Type.Boolean()),
-  failureDestination: Type.Optional(CronFailureAlertSchema),
+  failureDestination: Type.Optional(CronFailureDestinationSchema),
 };
 
 const CronDeliveryNoopSchema = Type.Object(