test(svg): add a ghost-city mode integration test in route (JhaSourav07#633)

## Description

Fixes JhaSourav07#380

## Pillar

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

## Visual Preview
No visual changes, just adding tests.

## 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.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord server for faster
collaboration, mentorship, and PR support.

Author: JhaSourav07

app/api/streak/route.test.ts

@@ -472,4 +472,24 @@ describe('GET /api/streak', () => {
       expect(response.headers.get('Cache-Control')).toBe('no-cache, no-store, must-revalidate');
     });
   });
+
+  describe('Ghost City Mode (route integration)', () => {
+    it('returns ghost city SVG when user has 0 total contributions', async () => {
+      const emptyCalendar: ContributionCalendar = {
+        totalContributions: 0,
+        weeks: [
+          {
+            contributionDays: [{ contributionCount: 0, date: '2024-06-10' }],
+          },
+        ],
+      };
+
+      vi.mocked(fetchGitHubContributions).mockResolvedValue(emptyCalendar);
+      const response = await GET(makeRequest({ user: 'octocat' }));
+      const body = await response.text();
+
+      expect(body).toContain('stroke-width="0.5"');
+      expect(body).toContain('stroke-opacity="0.3"');
+    });
+  });
 });