test(github): add bounds check for extreme values in generateAchievements (JhaSourav07#796)

## Description Fixes : JhaSourav07#692 

Adds a test to assert that the `generateAchievements` function correctly
clamps extreme progress values between 0 and 100, and verifies that the
progress value is finite.

## Pillar

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

## Visual Preview

N/A - This PR only adds automated tests and does not affect the SVG
output visually.

## 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.
- [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord server for faster
collaboration, mentorship, and PR support.

Author: JhaSourav07

lib/github.test.ts

@@ -538,6 +538,16 @@ describe('generateAchievements', () => {
 
     expect(unlocked.some((a) => a.title === '100 Day Streak')).toBe(false);
   });
+
+  it('caps progress between 0 and 100 for extreme values', () => {
+    const achievements = generateAchievements(999999, 999999);
+
+    for (const item of achievements) {
+      expect(Number.isFinite(item.progress)).toBe(true);
+      expect(item.progress).toBeGreaterThanOrEqual(0);
+      expect(item.progress).toBeLessThanOrEqual(100);
+    }
+  });
 });
 
 describe('validateGitHubUsername', () => {