test(validate-user): add comprehensive unit tests for GitHub user validation and caching (JhaSourav07#3162)

## Description

Adds a dedicated test suite for services/github/validate-user.ts to
verify GitHub user validation, cache behavior, and error handling.

The validator is responsible for preventing unnecessary GitHub API
requests by caching validation results and distinguishing between
permanent failures (user not found) and temporary API errors. These
tests ensure that behavior remains reliable and regression-free.

## Changes
Added
services/github/validate-user.test.ts
## Test Coverage

Implemented 5 unit tests covering:

- Successful validation of an existing GitHub user.
- Returning false when GitHub reports that a user does not exist.
- Caching successful validation results to avoid repeated API calls.
- Caching negative validation results for nonexistent users.
- Re-throwing temporary API errors without caching them.

Fixes JhaSourav07#2298 

## Pillar

- [x] 📐 Pillar 2 — Geometric SVG Improvement
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## 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): ...`).
- [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 (no raw elements, smooth animations, correct fonts).
- [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

services/github/validate-user.test.ts

@@ -0,0 +1,64 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { gitHubUserValidator } from './validate-user';
+import { fetchUserProfile } from '../../lib/github';
+
+vi.mock('../../lib/github', () => ({
+  fetchUserProfile: vi.fn(),
+}));
+
+describe('GitHubUserValidator', () => {
+  beforeEach(() => {
+    gitHubUserValidator.reset();
+    vi.clearAllMocks();
+  });
+
+  it('returns true for an existing GitHub user', async () => {
+    vi.mocked(fetchUserProfile).mockResolvedValue({} as never);
+
+    const result = await gitHubUserValidator.validateUser('octocat');
+
+    expect(result).toBe(true);
+    expect(fetchUserProfile).toHaveBeenCalledTimes(1);
+  });
+
+  it('returns false when GitHub reports user not found', async () => {
+    vi.mocked(fetchUserProfile).mockRejectedValue(new Error('User not found'));
+
+    const result = await gitHubUserValidator.validateUser('missing-user');
+
+    expect(result).toBe(false);
+    expect(fetchUserProfile).toHaveBeenCalledTimes(1);
+  });
+
+  it('caches successful validation results', async () => {
+    vi.mocked(fetchUserProfile).mockResolvedValue({} as never);
+
+    await gitHubUserValidator.validateUser('octocat');
+    await gitHubUserValidator.validateUser('octocat');
+
+    expect(fetchUserProfile).toHaveBeenCalledTimes(1);
+  });
+
+  it('caches negative validation results', async () => {
+    vi.mocked(fetchUserProfile).mockRejectedValue(new Error('User not found'));
+
+    await gitHubUserValidator.validateUser('ghost-user');
+    await gitHubUserValidator.validateUser('ghost-user');
+
+    expect(fetchUserProfile).toHaveBeenCalledTimes(1);
+  });
+
+  it('rethrows temporary GitHub API errors without caching', async () => {
+    vi.mocked(fetchUserProfile).mockRejectedValue(new Error('GitHub API rate limit exceeded'));
+
+    await expect(gitHubUserValidator.validateUser('octocat')).rejects.toThrow(
+      'GitHub API rate limit exceeded'
+    );
+
+    await expect(gitHubUserValidator.validateUser('octocat')).rejects.toThrow(
+      'GitHub API rate limit exceeded'
+    );
+
+    expect(fetchUserProfile).toHaveBeenCalledTimes(2);
+  });
+});