test(cache): verify Date instance values (JhaSourav07#1926)

## Description

Fixes JhaSourav07#1407

Added a TTLCache test to verify that Date instance values are preserved
correctly when stored and retrieved from the cache.

## 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:

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* [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.,
`feat(themes): ...`, `fix(calculate): ...`).
* [ ] I have updated `README.md` if I added a new theme or URL
parameter.
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* [x] I have made sure that I have only one commit to merge in this PR.
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Author: JhaSourav07

lib/cache.test.ts

@@ -402,6 +402,20 @@ describe('TTLCache', () => {
       expect(cache.get('tags')).toEqual(tags);
       cache.destroy();
     });
+    it('preserves Date instance values in TTLCache', () => {
+      const cache = new TTLCache<Date>();
+
+      const date = new Date('2026-05-31T00:00:00.000Z');
+
+      cache.set('created-at', date, 60_000);
+
+      const cached = cache.get('created-at');
+
+      expect(cached).toBeInstanceOf(Date);
+      expect(cached?.toISOString()).toBe(date.toISOString());
+
+      cache.destroy();
+    });
 
     it('stores and retrieves nested object values', () => {
       const cache = new TTLCache<{