test(api): add schema-level validation test for invalid accent hex color (JhaSourav07#1891)

## Description

Added schema-level validation tests targeting the `?accent=` parameter
under HEX color validation check. The tests verify that passing an
invalid hex value like `#ZZZZZZZ` is correctly rejected with a 400 Bad
Request, and that valid hex colors are accepted.

Changes
- Added a new test suite in `lib/validations.test.ts` targeting accent
HEX color validation
- Added a route-level test in `app/api/streak/route.test.ts` asserting
400 is returned for invalid accent hex

Fixes JhaSourav07#1450

## 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)

## Checklist

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

Author: JhaSourav07

app/api/streak/route.test.ts

@@ -648,6 +648,12 @@ describe('GET /api/streak', () => {
     it('does not crash when an invalid text color is provided', async () => {
       const response = await GET(makeRequest({ user: 'octocat', text: 'notacolor' }));
 
+      expect(response.status).toBe(400);
+    });
+    it('returns 400 when an invalid hex color is passed as accent', async () => {
+      // #ZZZZZZZ contains non-hex characters — schema must reject it with 400
+      const response = await GET(makeRequest({ user: 'octocat', accent: '#ZZZZZZZ' }));
+
       expect(response.status).toBe(400);
     });
   });

lib/validations.test.ts

@@ -695,23 +695,37 @@ describe('streakParamsSchema — view fallback behavior', () => {
   });
 });
 
-/* ==========================================================================
- * DATE RANGE BOUNDARY ROBUSTNESS (VARIATION 1)
- * ========================================================================== */
+describe('streakParamsSchema — accent parameter HEX color validation', () => {
+  it('rejects an invalid hex color like "#ZZZZZZZ" for accent', () => {
+    // #ZZZZZZZ contains non-hex characters — must fail schema validation
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      accent: '#ZZZZZZZ',
+    });
+
+    expect(result.success).toBe(false);
+  });
+
+  it('accepts a valid 6-character hex color for accent', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      accent: 'ff0000',
+    });
+
+    expect(result.success).toBe(true);
+  });
+});
 
 describe('streakParamsSchema — Date Range Boundary Robustness (Variation 1)', () => {
   it('should process validation safely and fallback when partial or missing year parameters are passed', () => {
-    // Arrange: Provide a mock payload missing a full YYYY format sequence
     const partialYearPayload = {
       user: 'octocat',
       from: '05-12',
       to: '05-30',
     };
 
-    // Act: Pass the object through the validator schema matrix
     const result = streakParamsSchema.safeParse(partialYearPayload);
 
-    // Assert: The validator handles it safely using implicit date engine fallbacks
     expect(result.success).toBe(true);
     if (result.success) {
       expect(result.data.from).toBeDefined();
@@ -720,15 +734,12 @@ describe('streakParamsSchema — Date Range Boundary Robustness (Variation 1)',
   });
 
   it('should pass cleanly and fallback to default ranges when date bounds are completely omitted', () => {
-    // Arrange: Pass only the bare minimum required parameters
     const minimalPayload = {
       user: 'octocat',
     };
 
-    // Act
     const result = streakParamsSchema.safeParse(minimalPayload);
 
-    // Assert: Verify that omitted range options return undefined to use downstream defaults smoothly
     expect(result.success).toBe(true);
     if (result.success) {
       expect(result.data.from).toBeUndefined();