test(middleware): add rate limit tests (JhaSourav07#1240)

## Description

Fixes JhaSourav07#1020

Added test coverage for middleware rate limiting behavior.

### Test Coverage Added

* Verifies `NextResponse.next()` is called when rate limiting succeeds.
* Verifies a `429` response is returned when rate limiting fails.
* Verifies the error response body contains `Too many requests`.
* Verifies successful responses include `X-RateLimit-Limit`.
* Verifies successful responses include `X-RateLimit-Remaining`.

## Pillar

* [ ] 🎨 Pillar 1 — New Theme Design
* [ ] 📐 Pillar 2 — Geometric SVG Improvement
* [ ] 🕐 Pillar 3 — Timezone Logic Optimization
* [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

Not applicable — test-only change.

## Checklist before requesting a review:

* [x] I have read the `CONTRIBUTING.md` file.
* [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
* [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
* [x] My commits follow the Conventional Commits format.
* [ ] I have updated `README.md` if I added a new theme or URL
parameter.
* [x] I have starred the repo.
* [x] I have made sure that I have only one commit to merge in this PR.
* [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard.
* [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

middleware.test.ts

@@ -0,0 +1,88 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { NextRequest, NextResponse } from 'next/server';
+import { middleware } from './middleware';
+import { rateLimit } from '@/lib/rate-limit';
+
+vi.mock('@/lib/rate-limit', () => ({
+  rateLimit: vi.fn(),
+}));
+
+describe('middleware', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('calls NextResponse.next when rate limit succeeds', () => {
+    vi.mocked(rateLimit).mockReturnValue({
+      success: true,
+      limit: 60,
+      remaining: 59,
+      reset: 123456789,
+    });
+
+    const nextSpy = vi.spyOn(NextResponse, 'next');
+
+    const request = new NextRequest('http://localhost:3000/api/streak?user=octocat');
+    middleware(request);
+
+    expect(nextSpy).toHaveBeenCalled();
+  });
+
+  it('returns 429 when rate limit fails', () => {
+    vi.mocked(rateLimit).mockReturnValue({
+      success: false,
+      limit: 60,
+      remaining: 0,
+      reset: 123456789,
+    });
+
+    const request = new NextRequest('http://localhost:3000/api/streak?user=octocat');
+    const response = middleware(request);
+
+    expect(response.status).toBe(429);
+  });
+
+  it('returns too many requests error body when rate limit fails', async () => {
+    vi.mocked(rateLimit).mockReturnValue({
+      success: false,
+      limit: 60,
+      remaining: 0,
+      reset: 123456789,
+    });
+
+    const request = new NextRequest('http://localhost:3000/api/streak?user=octocat');
+    const response = middleware(request);
+
+    await expect(response.json()).resolves.toEqual({
+      error: 'Too many requests',
+    });
+  });
+
+  it('sets X-RateLimit-Limit header when rate limit succeeds', () => {
+    vi.mocked(rateLimit).mockReturnValue({
+      success: true,
+      limit: 60,
+      remaining: 59,
+      reset: 123456789,
+    });
+
+    const request = new NextRequest('http://localhost:3000/api/streak?user=octocat');
+    const response = middleware(request);
+
+    expect(response.headers.get('X-RateLimit-Limit')).toBe('60');
+  });
+
+  it('sets X-RateLimit-Remaining header when rate limit succeeds', () => {
+    vi.mocked(rateLimit).mockReturnValue({
+      success: true,
+      limit: 60,
+      remaining: 59,
+      reset: 123456789,
+    });
+
+    const request = new NextRequest('http://localhost:3000/api/streak?user=octocat');
+    const response = middleware(request);
+
+    expect(response.headers.get('X-RateLimit-Remaining')).toBe('59');
+  });
+});