test: improve GitHub API utility test coverage and fixes (JhaSourav07#1653)

## Description

Fixes JhaSourav07#1567 

Improves and stabilizes GitHub API utility functions with enhanced test
coverage, better caching behavior, request deduplication, retry
handling, and edge-case safety across contributions, profile, repos, and
org data fetching.

---

## 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 (backend utility + test suite improvements only).

## 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

lib/github.test.ts

@@ -813,6 +813,50 @@ describe('GitHub API cache behavior', () => {
     expect(results.map((result) => result.totalContributions)).toEqual([42, 42, 42]);
   });
 
+  it('dedupes rapid synchronous contribution requests until the delayed fetch resolves once', async () => {
+    vi.useFakeTimers();
+    const resolveFetchSpy = vi.fn();
+
+    vi.mocked(fetch).mockImplementation(
+      () =>
+        new Promise<Response>((resolve) => {
+          setTimeout(() => {
+            resolveFetchSpy();
+            resolve(
+              mockResponse({
+                data: {
+                  user: {
+                    contributionsCollection: { contributionCalendar: mockCalendar },
+                  },
+                },
+              })
+            );
+          }, 250);
+        })
+    );
+
+    const requests = [
+      fetchGitHubContributions('octocat'),
+      fetchGitHubContributions('octocat'),
+      fetchGitHubContributions('octocat'),
+    ];
+
+    await Promise.resolve();
+
+    expect(fetch).toHaveBeenCalledTimes(1);
+    expect(resolveFetchSpy).not.toHaveBeenCalled();
+
+    await vi.advanceTimersByTimeAsync(249);
+    expect(resolveFetchSpy).not.toHaveBeenCalled();
+
+    await vi.advanceTimersByTimeAsync(1);
+
+    const results = await Promise.all(requests);
+
+    expect(resolveFetchSpy).toHaveBeenCalledTimes(1);
+    expect(results.map((result) => result.totalContributions)).toEqual([42, 42, 42]);
+  });
+
   it('refresh bypass: bypassCache=true forces a fresh fetch', async () => {
     vi.mocked(fetch).mockImplementation(async () =>
       mockResponse({