test(api): Add contract tests for API response schemas (#36)

tests/contract/api-schemas.test.js

@@ -0,0 +1,53 @@
+/**
+ * @fileoverview API Response Schema Contract Tests
+ * Validates that API responses adhere to expected data structures.
+ */
+
+const request = require('supertest');
+const { createApp } = require('../../src/app');
+
+describe('API Contract Tests - Response Schemas', () => {
+  let app;
+
+  beforeAll(() => {
+    app = createApp();
+  });
+
+  it('should match the GET /health response schema', async () => {
+    const res = await request(app).get('/health');
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual(
+      expect.objectContaining({
+        status: expect.any(String),
+        service: expect.any(String),
+        version: expect.any(String),
+        timestamp: expect.any(String),
+      })
+    );
+  });
+
+  it('should match the GET /api/invoices response schema', async () => {
+    const res = await request(app).get('/api/invoices');
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual(
+      expect.objectContaining({
+        data: expect.any(Array),
+        message: expect.any(String),
+      })
+    );
+  });
+
+  it('should match the POST /api/invoices response schema', async () => {
+    const res = await request(app).post('/api/invoices').send({});
+    expect(res.status).toBe(201);
+    expect(res.body).toEqual(
+      expect.objectContaining({
+        data: expect.objectContaining({
+          id: expect.any(String),
+          status: expect.any(String),
+        }),
+        message: expect.any(String),
+      })
+    );
+  });
+});