test(api): verify stale-while-revalidate=86400 in cache header (JhaSourav07#814)

## Description

Fixes JhaSourav07#704

## 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 - Test only, no visual changes

## 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=sandesh-861`).
- [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).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

app/api/streak/route.test.ts

@@ -826,4 +826,18 @@ describe('GET /api/streak', () => {
       expect(body).toContain('</svg>');
     });
   });
+
+  describe('stale-while-revalidate cache header', () => {
+    it('contains stale-while-revalidate=86400 for normal request', async () => {
+      const response = await GET(makeRequest({ user: 'octocat' }));
+
+      expect(response.headers.get('Cache-Control')).toContain('stale-while-revalidate=86400');
+    });
+
+    it('does NOT contain stale-while-revalidate when ?refresh=true', async () => {
+      const response = await GET(makeRequest({ user: 'octocat', refresh: 'true' }));
+
+      expect(response.headers.get('Cache-Control')).not.toContain('stale-while-revalidate=86400');
+    });
+  });
 });