test: add invalid bg hex validation coverage (JhaSourav07#3281)

## Description

Adds validation coverage for invalid `bg` hex color input.

Fixes JhaSourav07#3239

## 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 change, no visual output changes.

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally (`npm run test --
app/api/streak/route.test.ts`).
- [ ] 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.
- [ ] I have starred 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).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

app/api/streak/route.test.ts

@@ -943,7 +943,11 @@ describe('GET /api/streak', () => {
       expect(body.details).not.toBeNull();
     });
   });
+  it('returns 400 when an invalid hex color is passed as bg', async () => {
+    const response = await GET(makeRequest({ user: 'octocat', bg: '#ZZZZZZ' }));
 
+    expect(response.status).toBe(400);
+  });
   describe('hide parameters', () => {
     it('removes the username title when hide_title=true', async () => {
       const response = await GET(makeRequest({ user: 'octocat', hide_title: 'true' }));