test: add streakParamsSchema user validation tests (JhaSourav07#850)

## Description
Added unit tests for `streakParamsSchema` user field validation.

### Changes Made
- Added success test for valid user
- Added failure test for omitted user
- Added failure test for empty user
- Added failure test for username exceeding 39 characters
- Added failure test for invalid username characters

Closes JhaSourav07#659

---

## Pillar
- [x] Tests
- [ ] Bug Fix
- [ ] Feature
- [ ] Documentation
- [ ] Refactor

---

## Checklist
- [x] My code follows the project style guidelines
- [x] I have tested my changes locally
- [x] All tests are passing
- [x] I have linked the related issue
- [x] This PR is ready for review

Author: JhaSourav07

lib/validations.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest';
-import { githubParamsSchema } from './validations';
+import { githubParamsSchema, streakParamsSchema } from './validations';
 
 describe('githubParamsSchema', () => {
   it('should pass when username is valid', () => {
@@ -51,3 +51,54 @@ describe('githubParamsSchema', () => {
     }
   });
 });
+describe('streakParamsSchema user validation', () => {
+  it('should pass when user is valid', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octocat',
+    });
+
+    expect(result.success).toBe(true);
+  });
+
+  it('should fail when user is omitted', () => {
+    const result = streakParamsSchema.safeParse({});
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      expect(result.error.issues[0]?.message).toBe('Missing user parameter');
+    }
+  });
+
+  it('should fail when user is empty', () => {
+    const result = streakParamsSchema.safeParse({
+      user: '',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      expect(result.error.issues[0]?.message).toBe('Missing user parameter');
+    }
+  });
+
+  it('should fail when user exceeds 39 characters', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'a'.repeat(40),
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      expect(result.error.issues[0]?.message).toBe('GitHub username cannot exceed 39 characters');
+    }
+  });
+
+  it('should fail when user has invalid characters', () => {
+    const result = streakParamsSchema.safeParse({
+      user: 'octo_cat',
+    });
+
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      expect(result.error.issues[0]?.message).toBe('Invalid GitHub username');
+    }
+  });
+});