test(api validation): check query validation boundaries for ?date= parameter (Variation 4) (JhaSourav07#1959)

## Description

Fixes JhaSourav07#1459

This PR adds query validation boundaries for the `?date=` parameter in
`streakParamsSchema` to return a 400 Bad Request if an unsupported value
is supplied. It also adds unit tests and integration tests.

## Pillar

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

## Visual Preview

None (Validation changes)

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally.
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors.
- [x] My commits follow the Conventional Commits format.
- [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.

Author: JhaSourav07

app/api/streak/route.test.ts

@@ -672,6 +672,14 @@ describe('GET /api/streak', () => {
         expect(response.status).toBe(400);
         expect(body.details.fieldErrors.date[0]).toContain('Invalid "date" format');
       });
+
+      it('returns 400 when an invalid ISO8601 calendar date format like "2026-15-40" is supplied (Variation 4)', async () => {
+        const response = await GET(makeRequest({ user: 'octocat', date: '2026-15-40' }));
+        const body = await response.json();
+
+        expect(response.status).toBe(400);
+        expect(body.details.fieldErrors.date[0]).toContain('Invalid "date" format. Use ISO 8601.');
+      });
     });
   });
 

lib/validations.test.ts

@@ -1090,3 +1090,31 @@ describe('streakParamsSchema user maxLength validation boundaries (Variation 3)'
     expect(parseResult.success).toBe(true);
   });
 });
+
+/* ==========================================================================
+ * DATE PARAMETER — QUERY VALIDATION BOUNDARIES (VARIATION 4)
+ * ========================================================================== */
+
+describe('streakParamsSchema — date query validation boundaries (Variation 4)', () => {
+  it('rejects an invalid date format like "2026-15-40"', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      date: '2026-15-40',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      const messages = result.error.issues.map((i) => i.message).join(' ');
+      expect(messages).toContain('Invalid "date" format. Use ISO 8601.');
+    }
+  });
+
+  it('accepts a valid ISO8601 date', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      date: '2026-05-30',
+    });
+
+    expect(result.success).toBe(true);
+  });
+});