test(cache): add oversized cache key boundary test (JhaSourav07#1935)

## Description

Fixes JhaSourav07#1393

Added a boundary validation test for oversized cache keys in
`lib/cache.test.ts`.

This PR verifies that extremely large cache keys (`'a'.repeat(20000)`)
are handled safely without crashes or unexpected behavior during cache
operations.

## Pillar

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

## Visual Preview

N/A — this PR only adds a cache boundary validation test.

## 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.,
`feat(themes): ...`, `fix(calculate): ...`).
- [ ] 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 (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/cache.test.ts

@@ -541,6 +541,20 @@ describe('TTLCache', () => {
 
       cache.destroy();
     });
+
+    it('handles oversized cache keys safely', () => {
+      const cache = new TTLCache<string>();
+
+      const oversizedKey = 'a'.repeat(20000);
+
+      expect(() => {
+        cache.set(oversizedKey, 'large-key-value', 60_000);
+      }).not.toThrow();
+
+      expect(cache.get(oversizedKey)).toBe('large-key-value');
+
+      cache.destroy();
+    });
   });
 });