test(cache): verify TTLCache behavior for Date instance values (Variation 1) (JhaSourav07#1397) (JhaSourav07#2195)

## Description

Fixes JhaSourav07#1397 

## Pillar

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

## Visual Preview

## 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 (e.g.,
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--- 

I have also run the tests locally and verified them - 

<img width="950" height="142" alt="Screenshot 2026-06-01 011528"
src="https://github.com/user-attachments/assets/f61ac948-8ae5-4e17-b80b-f5db9ba3e68b"
/>

Author: JhaSourav07

lib/cache.test.ts

@@ -447,6 +447,51 @@ describe('TTLCache', () => {
       cache.destroy();
     });
 
+    it('preserves Date instance with current timestamp (new Date())', () => {
+      const cache = new TTLCache<Date>();
+
+      const now = new Date();
+
+      cache.set('current-date', now, 60_000);
+
+      const cached = cache.get('current-date');
+
+      expect(cached).toBeInstanceOf(Date);
+      expect(cached?.getTime()).toBe(now.getTime());
+      expect(cached?.toISOString()).toBe(now.toISOString());
+
+      cache.destroy();
+    });
+
+    it('preserves Date instance nested in object with mixed types', () => {
+      const cache = new TTLCache<{
+        id: number;
+        name: string;
+        created: Date;
+        isActive: boolean;
+      }>();
+
+      const created = new Date('2024-03-15T10:30:45.123Z');
+      const data = {
+        id: 42,
+        name: 'Test Event',
+        created: created,
+        isActive: true,
+      };
+
+      cache.set('event', data, 60_000);
+
+      const cached = cache.get('event');
+
+      expect(cached?.id).toBe(42);
+      expect(cached?.name).toBe('Test Event');
+      expect(cached?.isActive).toBe(true);
+      expect(cached?.created).toBeInstanceOf(Date);
+      expect(cached?.created.toISOString()).toBe(created.toISOString());
+
+      cache.destroy();
+    });
+
     it('stores and retrieves nested object values', () => {
       const cache = new TTLCache<{
         user: { id: number; name: string };