fix(bot): reject layouts with empty clusters

packages/backend/src/bot/bot.service.spec.ts

@@ -320,6 +320,21 @@ describe('BotService', () => {
     await expect(service.update(body)).rejects.toBeInstanceOf(BadRequestException);
   });
 
+  it('should reject layout with an empty cluster', async () => {
+    const body: UpdateBotDto = {
+      layout: [[0, 1], [], [2]],
+    };
+
+    (prismaService.bot.findFirst as jest.Mock).mockResolvedValueOnce({
+      id: HALLMASTER_BOT_ID,
+      totalShards: 0,
+      clusters: [],
+      dockerImage: MOCK_DOCKER_IMAGE,
+    });
+
+    await expect(service.update(body)).rejects.toBeInstanceOf(BadRequestException);
+  });
+
   it('should throw NotFoundException when updating non-existent bot', async () => {
     const body: UpdateBotDto = {
       layout: [

packages/backend/src/bot/bot.service.ts

@@ -45,6 +45,12 @@ export class BotService {
   }
 
   private validateLayout(layout: number[][]): void {
+    for (const [index, cluster] of layout.entries()) {
+      if (cluster.length === 0) {
+        throw new BadRequestException(`Cluster at index ${index} is empty.`);
+      }
+    }
+
     const allShardIds = layout.flat();
     const uniqueShardIds = new Set(allShardIds);
     const totalShards = allShardIds.length;

packages/backend/src/bot/dto/update-bot.dto.ts

@@ -4,7 +4,7 @@ import { z } from 'zod';
 import { CreateBotDockerImageSchema, CreateBotSchema } from './create-bot.dto.js';
 
 export const UpdateBotSchema = CreateBotSchema.extend({
-  layout: z.array(z.array(z.number().nonnegative())).min(1).meta({
+  layout: z.array(z.array(z.number().nonnegative()).min(1)).min(1).meta({
     description:
       'The cluster layout. Each element is an array of shard IDs assigned to that cluster. Example: [[0, 1, 2], [3]] creates 2 clusters with 4 total shards.',
   }),