test(validate-user): add empty input fallback coverage (JhaSourav07#3048)

## Description

Fixes JhaSourav07#2953 

Added isolated unit tests for `services/github/validate-user.ts`
focusing on edge cases, empty inputs, cache behavior, and fallback
handling.

### Changes
- Added `services/github/validate-user.empty-fallback.test.ts`
- Verified behavior when empty usernames are provided
- Tested whitespace-only username handling
- Confirmed missing users are cached correctly to avoid repeated API
calls
- Verified username normalization (case-insensitive and trimmed input)
uses cached results
- Ensured unexpected API/service failures are rethrown instead of being
silently swallowed
- Improved coverage around empty inputs, missing users, and fallback
execution paths

## 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 changes)

## 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 starred 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 SVG changes made).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

services/github/validate-user.empty-fallback.test.ts

@@ -0,0 +1,63 @@
+// services/github/validate-user.empty-fallback.test.ts
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('../../lib/github', () => ({
+  fetchUserProfile: vi.fn(),
+}));
+
+import { fetchUserProfile } from '../../lib/github';
+import { gitHubUserValidator } from './validate-user';
+
+describe('GitHubUserValidator Empty & Missing Input Fallbacks', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    gitHubUserValidator.reset();
+  });
+
+  it('throws for empty usernames', async () => {
+    vi.mocked(fetchUserProfile).mockRejectedValue(new Error('User not found'));
+
+    await expect(gitHubUserValidator.validateUser('')).rejects.toThrow('Cache key cannot be empty');
+  });
+
+  it('throws for whitespace-only usernames', async () => {
+    vi.mocked(fetchUserProfile).mockRejectedValue(new Error('User not found'));
+
+    await expect(gitHubUserValidator.validateUser('     ')).rejects.toThrow(
+      'Cache key cannot be empty'
+    );
+  });
+
+  it('caches negative results for missing users', async () => {
+    vi.mocked(fetchUserProfile).mockRejectedValue(new Error('User not found'));
+
+    const first = await gitHubUserValidator.validateUser('missing-user');
+    const second = await gitHubUserValidator.validateUser('missing-user');
+
+    expect(first).toBe(false);
+    expect(second).toBe(false);
+
+    expect(fetchUserProfile).toHaveBeenCalledTimes(1);
+  });
+
+  it('returns cached result for usernames with different casing and spacing', async () => {
+    vi.mocked(fetchUserProfile).mockResolvedValue({} as never);
+
+    const first = await gitHubUserValidator.validateUser('Ganesh');
+    const second = await gitHubUserValidator.validateUser('  ganesh  ');
+
+    expect(first).toBe(true);
+    expect(second).toBe(true);
+
+    expect(fetchUserProfile).toHaveBeenCalledTimes(1);
+  });
+
+  it('rethrows unexpected service errors instead of masking them', async () => {
+    vi.mocked(fetchUserProfile).mockRejectedValue(new Error('GitHub API unavailable'));
+
+    await expect(gitHubUserValidator.validateUser('ganesh')).rejects.toThrow(
+      'GitHub API unavailable'
+    );
+  });
+});