test(frontend): add vitest unit tests for the /api/health route handler

Follow-up to PR #87 (docker: add multi-stage frontend Dockerfile with Next.js standalone output) to ensure the App Router liveness endpoint matches the Docker HEALTHCHECK expectations in docker/frontend.Dockerfile.

- Assert GET returns status 200 and application/json content type

- Assert the JSON body has status: ok plus an ISO 8601 UTC timestamp

- Assert module-level dynamic = force-dynamic export so Next.js does not pre-render the route

Refs #87

Author: Moonwalker-rgb

Frontend/src/app/api/health/route.test.ts

@@ -0,0 +1,45 @@
+/**
+ * Tests for the GET /api/health liveness probe.
+ *
+ * Follow-up to PR #87 (`docker: add multi-stage frontend Dockerfile with
+ * Next.js standalone output`) so the Next.js App Router endpoint matches
+ * the Docker HEALTHCHECK expectations in `docker/frontend.Dockerfile`.
+ *
+ * The route is exercised by `wget --spider` against
+ * `http://127.0.0.1:${PORT}/api/health`, so we assert only the
+ * duck-typed Web Response shape — no `instanceof` checks against Next.js
+ * internal classes, because vitest's module-resolution surface differs
+ * from the production runtime and `instanceof NextResponse` is not
+ * guaranteed to survive that.
+ */
+import { describe, it, expect } from 'vitest'
+import { GET, dynamic } from './route'
+
+describe('GET /api/health', () => {
+  it('returns status 200', () => {
+    const response = GET()
+    expect(response.status).toBe(200)
+  })
+
+  it('returns application/json content type', () => {
+    const response = GET()
+    expect(response.headers.get('content-type')).toMatch(/application\/json/)
+  })
+
+  it('returns body with status: "ok" and an ISO 8601 timestamp', async () => {
+    const response = GET()
+    const body = await response.json()
+
+    expect(body.status).toBe('ok')
+    expect(typeof body.timestamp).toBe('string')
+    expect(new Date(body.timestamp).toString()).not.toBe('Invalid Date')
+    // Strict ISO 8601 UTC: 2024-05-04T12:34:56.789Z
+    expect(body.timestamp).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/)
+  })
+})
+
+describe('route module config', () => {
+  it('exports dynamic = "force-dynamic" so Next.js does not pre-render the route', () => {
+    expect(dynamic).toBe('force-dynamic')
+  })
+})