test(track-user): add uppercase username normalization test (JhaSourav07#949)

## Description
Added a new test case for uppercase username normalization in
`app/api/track-user/route.test.ts`.

### Changes made

* Added test for `GITHUB` username normalization
* Set `MONGODB_URI` environment variable in test setup
* Verified `User.findOneAndUpdate` is called with `{ username: 'github'
}`
* Ensured existing track-user tests continue to pass
* No functional changes

Fixes JhaSourav07#693

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## 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`).
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resolved all errors (CI will fail otherwise).
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Author: JhaSourav07

app/api/track-user/route.test.ts

@@ -122,6 +122,24 @@ describe('POST /api/track-user', () => {
       expect(data.bypassed).toBeUndefined();
     });
 
+    it('normalizes purely uppercase usernames to lowercase', async () => {
+      const response = await POST(makeRequest({ username: 'GITHUB' }));
+
+      expect(dbConnect).toHaveBeenCalled();
+
+      expect(User.updateOne).toHaveBeenCalledWith(
+        { username: 'github' },
+        { $setOnInsert: { username: 'github' } },
+        { upsert: true }
+      );
+
+      expect(response.status).toBe(200);
+
+      const data = await response.json();
+
+      expect(data.success).toBe(true);
+    });
+
     it('returns 500 when database connection fails', async () => {
       const consoleErrorSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
       vi.mocked(dbConnect).mockRejectedValueOnce(new Error('DB Down'));