test(displayName): add unit tests for displayName helper (JhaSourav07#1164)

## Description

Fixes JhaSourav07#1057

Added focused unit tests for the `displayName` helper in
`lib/github.ts`.

### Test Coverage Added

* Returns the user's name when present.
* Falls back to login when name is `null`.
* Falls back to login when name is an empty string.
* Falls back to login when name contains only whitespace.

## 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.
* [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 raw elements, smooth animations, correct fonts).
* [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: Vishvadharman

lib/github.test.ts

@@ -12,6 +12,7 @@ import {
   validateGitHubUsername,
   cacheKey,
   buildInsights,
+  displayName,
   fetchOrgMembers,
   getOrgDashboardData,
   getWrappedData,
@@ -805,13 +806,13 @@ describe('generateAchievements', () => {
     const achievements = generateAchievements(600, 10, 0, 0);
 
     const unlocked = achievements.filter((a) => a.isUnlocked);
+
     expect(unlocked.some((a) => a.title === '500 Contributions')).toBe(true);
     expect(unlocked.some((a) => a.title === 'Consistency King')).toBe(true);
     expect(unlocked.some((a) => a.title === '1000 Contributions')).toBe(false);
   });
 
   it('unlocks all achievements for max contribution and streak values', () => {
-    // Consistency King III requires 2000, streak needs 100, weekend needs 10, polyglot needs 5
     const achievements = generateAchievements(2001, 101, 11, 6);
 
     expect(achievements.every((achievement) => achievement.isUnlocked === true)).toBe(true);
@@ -846,6 +847,36 @@ describe('generateAchievements', () => {
   });
 });
 
+describe('displayName', () => {
+  const makeProfile = (name: string | null) => ({
+    login: 'octocat',
+    name,
+    avatar_url: 'avatar.png',
+    public_repos: 0,
+    followers: 0,
+    following: 0,
+    created_at: '2020-01-01T00:00:00Z',
+    bio: null,
+    location: null,
+  });
+
+  it('returns the name when present', () => {
+    expect(displayName(makeProfile('The Octocat'))).toBe('The Octocat');
+  });
+
+  it('falls back to login when name is null', () => {
+    expect(displayName(makeProfile(null))).toBe('octocat');
+  });
+
+  it('falls back to login when name is empty', () => {
+    expect(displayName(makeProfile(''))).toBe('octocat');
+  });
+
+  it('falls back to login when name contains only whitespace', () => {
+    expect(displayName(makeProfile('   '))).toBe('octocat');
+  });
+});
+
 describe('validateGitHubUsername', () => {
   it('returns true for a valid username', () => {
     expect(validateGitHubUsername('valid-username-123')).toBe(true);