fix(ui): clearing a cron delivery destination in the Control UI silently keeps the old chat (#123171)

Blanking the 'To' field in the Automations delivery editor omitted the
key from the cron.update patch. The Gateway merges delivery patches
by key presence, so the stored destination survived and every future
run kept announcing to the old chat with no error. Send an explicit
null clear on update, exactly like the accountId field one line above
(same bug class fixed there in #105762). Webhook mode is unaffected:
its blank-URL validation rejects the form before submit.

ui/src/lib/cron/index.test.ts

@@ -680,6 +680,34 @@ describe("cron controller", () => {
     });
   });
 
+  it("sends null delivery.to in cron.update to clear a persisted destination", async () => {
+    const job = createCronJob({
+      id: "job-clear-to",
+      name: "clear to",
+      delivery: { mode: "announce", channel: "telegram", to: "12345" },
+    });
+    const { submit } = createCronSubmitHarness(job.id, {
+      method: "cron.update",
+      jobs: [job],
+      form: {
+        name: "clear to",
+        scheduleKind: "cron",
+        cronExpr: "0 * * * *",
+        wakeMode: "next-heartbeat",
+        payloadText: "run",
+        deliveryMode: "announce",
+        deliveryTo: "   ",
+      },
+    });
+
+    const { call } = await submit();
+
+    expectRecordFields(requireRecord(requestPatch(call).delivery, "delivery"), {
+      mode: "announce",
+      to: null,
+    });
+  });
+
   it("maps a cron job into editable form fields", () => {
     const state = createState();
     const job = createCronJob({

ui/src/lib/cron/index.ts

@@ -1150,7 +1150,9 @@ export async function addCronJob(state: CronState): Promise<CronSaveResult> {
                     preserveLastOnUpdate: Boolean(editingJob?.delivery?.channel),
                   })
                 : undefined,
-            to: form.deliveryTo.trim() || undefined,
+            to:
+              form.deliveryTo.trim() ||
+              (selectedDeliveryMode === "announce" && editingJob?.delivery?.to ? null : undefined),
             accountId: deliveryAccountId,
             bestEffort: form.deliveryBestEffort,
             ...(form.deliveryThreadId !== undefined ? { threadId: form.deliveryThreadId } : {}),