test(api-validation): add query validation boundaries for ?accent= parameter(Variation 5) (JhaSourav07#2121)

## Description
Adds validation coverage for the `accent` query parameter.

### Changes

- Added schema-level test for invalid HEX color syntax (`#ZZZZZZ`)
- Added endpoint test to verify invalid accent values return HTTP 400
- Ensures malformed color inputs are rejected before reaching core logic

### Testing

- All test suites pass
- Streak endpoint validation verified

Fixes JhaSourav07#1466

## Pillar

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

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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`).
- [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.
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standard (no raw elements, smooth animations, correct fonts).
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Author: JhaSourav07

app/api/streak/route.test.ts

@@ -780,6 +780,16 @@ describe('GET /api/streak', () => {
 
       expect(response.status).toBe(400);
     });
+
+    it('returns 400 Bad Request for invalid color hex syntax targeting the ?accent= parameter', async () => {
+      const response = await GET(makeRequest({ user: 'octocat', accent: '#ZZZZZZ' }));
+
+      expect(response.status).toBe(400);
+
+      const body = await response.json();
+      expect(body.error).toBe('Invalid parameters');
+      expect(body.details).not.toBeNull();
+    });
   });
 
   describe('hide parameters', () => {

lib/validations.test.ts

@@ -814,6 +814,22 @@ describe('streakParamsSchema — accent parameter HEX color validation', () => {
     expect(result.success).toBe(false);
   });
 
+  it('rejects an invalid hex color like "#ZZZZZZ" for accent (Variation 5)', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      accent: '#ZZZZZZ',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      // This extra check ensures Variation 5 isn't just a duplicate,
+      // but a stricter validation check!
+      expect(result.error.issues[0]?.message).toContain(
+        'accent must be a valid 3 or 6 character hex color without #'
+      );
+    }
+  });
+
   it('rejects the invalid boundary hex color "#ZZZZZZ" for accent', () => {
     const result = streakParamsSchema.safeParse({
       user: 'octocat',