test(utils): check boundary robustness of rate limit TTL timers (Variation 3) (JhaSourav07#2229)

## Description
Adds a utility verification test to validate sliding time range behavior
and ensure keys expire exactly at the configured window boundary.

### Changes Made

- Added 1 new test block for the utility function.
- Verified behavior across sliding time ranges.
- Confirmed that keys remain valid before the window limit and expire
exactly at the boundary.

### Testing

- Utility verification tests compile successfully.
- Existing test suite remains unaffected.

Fixes JhaSourav07#1571

## Pillar

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

## Visual Preview
<img width="1537" height="768" alt="image"
src="https://github.com/user-attachments/assets/a7e4ff33-e7b6-4e62-9d8c-9b8c1b3c5659"
/>


## 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): ...`).
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
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Author: JhaSourav07

utils/time.test.ts

@@ -264,6 +264,25 @@ describe('getSecondsUntilUTCMidnight — sliding window boundary robustness', ()
     vi.useRealTimers();
   });
 
+  it('verifies utility guarantees keys expire exactly at window limit across a sliding range', () => {
+    // Target inputs: Sliding time range approaching midnight in Asia/Kolkata (UTC+5:30)
+    // Local midnight happens at UTC 18:30:00
+    const slidingCases: [string, number][] = [
+      ['2024-06-15T17:30:00.000Z', 3600], // 1 hour before local midnight
+      ['2024-06-15T18:00:00.000Z', 1800], // 30 mins before local midnight
+      ['2024-06-15T18:29:59.000Z', 1], // 1 second before local midnight
+      ['2024-06-15T18:30:00.000Z', 86400], // Exactly local midnight (resets to full day)
+    ];
+
+    for (const [utcTime, expectedTTL] of slidingCases) {
+      vi.setSystemTime(new Date(utcTime));
+      const seconds = getSecondsUntilMidnightInTimezone('Asia/Kolkata');
+
+      // Assert that outputs match guarantees keys expire exactly at window limit
+      expect(seconds).toBe(expectedTTL);
+    }
+  });
+
   it('returns correct TTL across a sliding window of times approaching UTC midnight', () => {
     // Verifies that the utility guarantees keys expire exactly at the window limit.
     // Each entry is [UTC time string, expected seconds until next midnight].