Feat/grace range validation (JhaSourav07#3041)

## Description
Fixes JhaSourav07#1462 

Updated grace validation tests and aligned `validations.ts` to correctly
validate `grace` values between `0–7`.
The original issue focused on tests, but validation logic also required
updates so the schema behavior matched expected test outcomes.

### Changes made
* Added/updated tests for `grace` validation
* Ensured invalid `grace` values below `0` fail validation
* Updated validation logic in `validations.ts` to align with expected
schema behavior
* Verified schema behavior for valid and invalid inputs

## 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 (validation and test changes only)

## 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).
* [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

app/api/streak/route.test.ts

@@ -277,6 +277,24 @@ describe('GET /api/streak', () => {
       expect(fetchGitHubContributions).not.toHaveBeenCalled();
     });
 
+    it('returns 400 when grace is below the minimum value', async () => {
+      const response = await GET(
+        makeRequest({
+          user: 'octocat',
+          grace: '-1',
+        })
+      );
+
+      expect(response.status).toBe(400);
+
+      const body = await response.json();
+
+      expect(body.error).toBe('Invalid parameters');
+      expect(body.details.fieldErrors.grace[0]).toBe('grace must be an integer between 0 and 7');
+
+      expect(fetchGitHubContributions).not.toHaveBeenCalled();
+    });
+
     it('returns 400 for unsupported ?layout query parameter values (strict schema validation)', async () => {
       const response = await GET(
         new Request('http://localhost:3000/api/streak?user=octocat&layout=unsupported_layout')

lib/validations.test.ts

@@ -229,6 +229,20 @@ describe('streakParamsSchema', () => {
     }
   });
 
+  it('should fail when grace is below the minimum value of 0', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+      grace: '-1',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      expect(result.error.flatten().fieldErrors.grace?.[0]).toBe(
+        'grace must be an integer between 0 and 7'
+      );
+    }
+  });
+
   it('accepts a valid width value', () => {
     const result = streakParamsSchema.safeParse({
       user: 'octocat',

lib/validations.ts

@@ -280,13 +280,14 @@ const baseStreakParamsSchema = z.object({
     .optional()
     .refine(
       (val) => {
-        if (val === undefined) return true;
+        if (!val) return true;
         const parsed = Number(val);
         return !isNaN(parsed) && Number.isInteger(parsed) && parsed >= 0 && parsed <= 7;
       },
       { message: 'grace must be an integer between 0 and 7' }
     )
-    .transform((val) => (val === undefined ? 1 : Number(val))),
+    .transform(toGraceValue)
+    .default(1),
   mode: z.enum(['commits', 'loc']).catch('commits').default('commits'),
   repo: z.string().optional(),
   org: z