test(leaderboard): add error resilience coverage (JhaSourav07#3200)

## Description

Fixes JhaSourav07#2758

Adds focused Vitest coverage for hydration stability, exception safety,
and error resilience behavior in `components/Leaderboard.tsx`.

Tests verify:

* Valid contributor data renders without runtime errors
* Empty contributor arrays render safely
* Empty-state rendering remains hydration-safe
* Minimal contributor values do not crash rendering
* Repeated renders remain stable without exceptions

## Pillar

* [ ] 🎨 Pillar 1 — New Theme Design
* [ ] 📐 Pillar 2 — Geometric SVG Improvement
* [ ] 🕐 Pillar 3 — Timezone Logic Optimization
* [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

Not applicable — test-only change.

## Checklist before requesting a review:

* [x] I have read the `CONTRIBUTING.md` file.
* [ ] 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.
* [ ] 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

components/Leaderboard.error-resilience.test.tsx

@@ -0,0 +1,66 @@
+import { render } from '@testing-library/react';
+import type { ComponentProps, ReactNode } from 'react';
+import { describe, expect, it, vi } from 'vitest';
+import Leaderboard, { type Contributor } from './Leaderboard';
+
+vi.mock('next/image', () => ({
+  default: ({ alt = '', ...props }: ComponentProps<'img'>) => (
+    // eslint-disable-next-line @next/next/no-img-element
+    <img alt={alt} {...props} />
+  ),
+}));
+
+vi.mock('framer-motion', () => ({
+  motion: {
+    div: ({ children, ...props }: ComponentProps<'div'> & { children?: ReactNode }) => (
+      <div {...props}>{children}</div>
+    ),
+  },
+}));
+
+const contributors: Contributor[] = [
+  {
+    id: 1,
+    login: 'alice',
+    avatar_url: '/avatar.png',
+    contributions: 100,
+    html_url: 'https://github.com/alice',
+  },
+];
+
+describe('Leaderboard error resilience', () => {
+  it('renders without crashing with valid contributor data', () => {
+    expect(() => render(<Leaderboard contributors={contributors} />)).not.toThrow();
+  });
+
+  it('renders safely with an empty contributor array', () => {
+    expect(() => render(<Leaderboard contributors={[]} />)).not.toThrow();
+  });
+
+  it('maintains stable hydration-safe rendering for empty state', () => {
+    const { container } = render(<Leaderboard contributors={[]} />);
+
+    expect(container).toBeTruthy();
+    expect(container.firstChild).not.toBeNull();
+  });
+
+  it('does not throw when contributor list contains minimal values', () => {
+    const minimalContributors: Contributor[] = [
+      {
+        id: 1,
+        login: '',
+        avatar_url: '',
+        contributions: 0,
+        html_url: '',
+      },
+    ];
+
+    expect(() => render(<Leaderboard contributors={minimalContributors} />)).not.toThrow();
+  });
+
+  it('survives repeated renders without runtime exceptions', () => {
+    const { rerender } = render(<Leaderboard contributors={contributors} />);
+
+    expect(() => rerender(<Leaderboard contributors={contributors} />)).not.toThrow();
+  });
+});