test(RateLimiter): add unit tests for RateLimiter class (JhaSourav07#1216)

## Description
Fixes JhaSourav07#1022

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

## Changes
Added 4 unit tests for the `RateLimiter` class in
`lib/rate-limit.test.ts`:
- `check(ip)` returns `true` when within limit
- `check(ip)` returns `false` after exceeding limit
- Multiple IPs are tracked independently
- `check(ip)` returns `true` again after window reset

## Visual Preview
N/A — no visual changes.

## Checklist before requesting a review:
- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally (`npm run test` — 8/8 tests
pass).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors.
- [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.
- [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard.
- [ ] (Recommended) I joined the CommitPulse Discord community.

Author: JhaSourav07

lib/rate-limit.test.ts

@@ -1,5 +1,6 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { rateLimit } from './rate-limit';
+import { RateLimiter } from './rate-limit';
 
 describe('rateLimit', () => {
   beforeEach(() => {
@@ -60,3 +61,48 @@ describe('rateLimit', () => {
     expect(rateLimit(ip2, 60, 60000).success).toBe(true);
   });
 });
+
+describe('RateLimiter', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  it('allows requests within the limit', () => {
+    // Each check() within the limit should return true
+    const limiter = new RateLimiter(3, 60000);
+    expect(limiter.check('1.1.1.1')).toBe(true);
+    expect(limiter.check('1.1.1.1')).toBe(true);
+    expect(limiter.check('1.1.1.1')).toBe(true);
+  });
+
+  it('blocks requests after exceeding the limit', () => {
+    // 4th request should be denied when limit is 3
+    const limiter = new RateLimiter(3, 60000);
+    limiter.check('2.2.2.2');
+    limiter.check('2.2.2.2');
+    limiter.check('2.2.2.2');
+    expect(limiter.check('2.2.2.2')).toBe(false);
+  });
+
+  it('tracks multiple IPs independently', () => {
+    // Exhausting one IP's limit should not affect another IP
+    const limiter = new RateLimiter(2, 60000);
+    limiter.check('3.3.3.3');
+    limiter.check('3.3.3.3');
+    expect(limiter.check('3.3.3.3')).toBe(false);
+    expect(limiter.check('4.4.4.4')).toBe(true);
+  });
+
+  it('allows requests again after the window resets', () => {
+    // TTL expiry should clear the count, allowing the IP through again
+    const windowMs = 60000;
+    const limiter = new RateLimiter(2, windowMs);
+    limiter.check('5.5.5.5');
+    limiter.check('5.5.5.5');
+    expect(limiter.check('5.5.5.5')).toBe(false);
+
+    vi.advanceTimersByTime(windowMs + 1);
+
+    expect(limiter.check('5.5.5.5')).toBe(true);
+  });
+});