test(api validation): check query validation boundaries for ?theme= parameter (Variation 2) (JhaSourav07#1439) (JhaSourav07#2232)

## Description

Fixes JhaSourav07#1439 

## Pillar

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

## Visual Preview

## 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): ...`).
- [ ] 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 community for
contributor discussions, mentorship, and faster PR support.


--- 

I have run tests on them, and they are working perfectly fine - 

- lib/validations.test.ts
<img width="954" height="134" alt="Screenshot 2026-06-01 125734"
src="https://github.com/user-attachments/assets/c366e437-3bbd-473b-81ba-437e9a0e209d"
/>

- app/api/streak/route.test.ts
<img width="1021" height="130" alt="Screenshot 2026-06-01 125816"
src="https://github.com/user-attachments/assets/f333238c-a9aa-457b-bed7-0a4b55f42c6d"
/>

Author: JhaSourav07

app/api/streak/route.test.ts

@@ -259,6 +259,28 @@ describe('GET /api/streak', () => {
       expect(response.status).toBe(400);
     });
 
+    it('returns 400 when an invalid theme value is provided and lists allowed themes', async () => {
+      const response = await GET(
+        makeRequest({
+          user: 'octocat',
+          theme: 'nonexistent_theme_name',
+        })
+      );
+
+      expect(response.status).toBe(400);
+      const body = await response.json();
+      expect(body.error).toBe('Invalid parameters');
+      expect(body.details.fieldErrors.theme).toBeDefined();
+      const errorMessage = body.details.fieldErrors.theme[0];
+      expect(errorMessage).toContain('Invalid theme');
+      expect(errorMessage).toContain('Supported themes:');
+      expect(errorMessage).toContain('auto');
+      expect(errorMessage).toContain('random');
+      expect(errorMessage).toContain('dark');
+      expect(errorMessage).toContain('light');
+      expect(fetchGitHubContributions).not.toHaveBeenCalled();
+    });
+
     it('should return 200 OK and valid SVG when the optional repo query parameter is provided', async () => {
       // 1. Make request with both parameters present
       const response = await GET(makeRequest({ user: 'octocat', repo: 'commitpulse' }));

lib/validations.test.ts

@@ -850,6 +850,29 @@ describe('streakParamsSchema — theme validation', () => {
       expect(fieldError).toContain('neon');
     }
   });
+
+  it('should reject nonexistent_theme_name and verify allowed themes are listed in error', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      theme: 'nonexistent_theme_name',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      const fieldErrors = result.error.flatten().fieldErrors;
+      expect(fieldErrors.theme).toBeDefined();
+      const errorMessage = fieldErrors.theme?.[0];
+      expect(errorMessage).toContain('Invalid theme');
+      expect(errorMessage).toContain('Supported themes:');
+      expect(errorMessage).toContain('auto');
+      expect(errorMessage).toContain('random');
+      expect(errorMessage).toContain('dark');
+      expect(errorMessage).toContain('light');
+      expect(errorMessage).toContain('neon');
+      expect(errorMessage).toContain('github');
+      expect(errorMessage).toContain('dracula');
+    }
+  });
 });
 
 describe('streakParamsSchema — view fallback behavior', () => {