feat: add course AI agent backend

src/ai/ai-credential.service.spec.ts

@@ -0,0 +1,23 @@
+import { BadRequestException } from '@nestjs/common';
+import { ConfigService } from '@nestjs/config';
+
+import { AiCredentialService } from './ai-credential.service';
+
+describe('AiCredentialService', () => {
+  it('encrypts and decrypts an API key without storing plaintext', () => {
+    const config = { get: jest.fn().mockReturnValue('a-secure-test-secret-with-more-than-24-characters') } as unknown as ConfigService;
+    const service = new AiCredentialService(config);
+
+    const encrypted = service.encrypt('sk-test-secret');
+
+    expect(encrypted).not.toContain('sk-test-secret');
+    expect(service.decrypt(encrypted)).toBe('sk-test-secret');
+  });
+
+  it('rejects a short encryption secret', () => {
+    const config = { get: jest.fn().mockReturnValue('too-short') } as unknown as ConfigService;
+    const service = new AiCredentialService(config);
+
+    expect(() => service.encrypt('sk-test-secret')).toThrow(BadRequestException);
+  });
+});