test(api): harden assistant tool error handling

packages/api/src/router/notification.ts

@@ -619,6 +619,13 @@ export const notificationRouter = createTRPCRouter({
         senderId,
       );
 
+      if (recipientIds.length === 0) {
+        throw new TRPCError({
+          code: "BAD_REQUEST",
+          message: "No recipients matched the broadcast target.",
+        });
+      }
+
       // 4. Create individual notifications for each recipient
       const isTask = input.category === "TASK" || input.category === "APPROVAL";