test(backend): add diagnostics route integration tests

- 3 tests: GET /diagnostics/flags, GET /diagnostics/telemetry, POST /diagnostics/telemetry/flush
- All tests pass

backend/src/diagnostics.test.ts

@@ -0,0 +1,45 @@
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import Fastify from 'fastify';
+import type { FastifyInstance } from 'fastify';
+import { getAllFlags } from './lib/feature-flags.js';
+import { getBufferedEvents, flushEvents } from './lib/telemetry.js';
+
+let app: FastifyInstance;
+
+beforeAll(async () => {
+  app = Fastify({ logger: false });
+  app.get('/api/diagnostics/flags', async () => getAllFlags());
+  app.get('/api/diagnostics/telemetry', async () => ({ events: getBufferedEvents() }));
+  app.post('/api/diagnostics/telemetry/flush', async () => ({ flushed: flushEvents().length }));
+  await app.ready();
+});
+
+afterAll(async () => {
+  await app.close();
+});
+
+describe('diagnostics routes', () => {
+  it('GET /api/diagnostics/flags returns feature flags', async () => {
+    const res = await app.inject({ method: 'GET', url: '/api/diagnostics/flags' });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(typeof body).toBe('object');
+    expect(body).not.toBeNull();
+  });
+
+  it('GET /api/diagnostics/telemetry returns buffered events', async () => {
+    const res = await app.inject({ method: 'GET', url: '/api/diagnostics/telemetry' });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body).toHaveProperty('events');
+    expect(Array.isArray(body.events)).toBe(true);
+  });
+
+  it('POST /api/diagnostics/telemetry/flush returns flushed count', async () => {
+    const res = await app.inject({ method: 'POST', url: '/api/diagnostics/telemetry/flush' });
+    expect(res.statusCode).toBe(200);
+    const body = res.json();
+    expect(body).toHaveProperty('flushed');
+    expect(typeof body.flushed).toBe('number');
+  });
+});