fix: validate AI agent request text correctly

src/ai/dto/create-agent-plan.dto.spec.ts

@@ -0,0 +1,26 @@
+import 'reflect-metadata';
+
+import { validate } from 'class-validator';
+
+import { CreateAgentPlanDto } from './create-agent-plan.dto';
+
+describe('CreateAgentPlanDto', () => {
+  const createDto = (request: string): CreateAgentPlanDto => Object.assign(new CreateAgentPlanDto(), {
+    courseId: '019f3689-4fe2-7b93-a7c4-72e51fe3ec17',
+    week: 1,
+    request,
+    modules: [],
+  });
+
+  it('accepts a non-empty agent request', async () => {
+    const errors = await validate(createDto('지금 모듈 몇 개야?'));
+
+    expect(errors).toHaveLength(0);
+  });
+
+  it('rejects an empty agent request', async () => {
+    const errors = await validate(createDto(''));
+
+    expect(errors.some((error) => error.property === 'request')).toBe(true);
+  });
+});