test: add regression tests for SVG structural cleanliness and duplicate render blocks (JhaSourav07#2145)

## Description
Fixes JhaSourav07#2104

Added comprehensive regression tests to prevent malformed SVG structures
from being reintroduced. Tests verify:
- Exactly one root `<svg>` and closing `</svg>` tag (no nested wrappers)
- No duplicate `renderHeader` or `renderStyle` calls
- No duplicate `<style>` blocks
- Proper structural cleanliness

This ensures the SVG structural bug never resurfaces.
## Pillar
- [x] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [ ] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
GitHub theme badge now displays with `#238636` accent color, matching
the official documentation.

## 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=Pranav-IIITM&theme=github`).
- [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/svg/generator.test.ts

@@ -1675,6 +1675,50 @@ describe('deterministicRandom', () => {
   });
 });
 
+describe('SVG Structural Validity and Cleanliness', () => {
+  const mockStats: StreakStats = {
+    currentStreak: 5,
+    longestStreak: 10,
+    totalContributions: 100,
+    todayDate: '2024-06-12',
+  };
+  const mockCalendar = {
+    weeks: [
+      {
+        contributionDays: [
+          { contributionCount: 0, date: '2024-06-10' },
+          { contributionCount: 5, date: '2024-06-11' },
+          { contributionCount: 15, date: '2024-06-12' },
+        ],
+      },
+    ],
+  } as ContributionCalendar;
+
+  it('generateSVG output contains exactly one root <svg> element and exactly one closing </svg> tag', () => {
+    const svg = generateSVG(mockStats, { user: 'avi' } as unknown as BadgeParams, mockCalendar);
+    const openCount = (svg.match(/<svg/gi) || []).length;
+    const closeCount = (svg.match(/<\/svg>/gi) || []).length;
+    expect(openCount).toBe(1);
+    expect(closeCount).toBe(1);
+  });
+
+  it('generateSVG output contains exactly one style block', () => {
+    const svg = generateSVG(mockStats, { user: 'avi' } as unknown as BadgeParams, mockCalendar);
+    const styleOpenCount = (svg.match(/<style>/gi) || []).length;
+    const styleCloseCount = (svg.match(/<\/style>/gi) || []).length;
+    expect(styleOpenCount).toBe(1);
+    expect(styleCloseCount).toBe(1);
+  });
+
+  it('generateSVG does not contain duplicate renderHeader or renderStyle outputs', () => {
+    const svg = generateSVG(mockStats, { user: 'avi' } as unknown as BadgeParams, mockCalendar);
+    const titleCount = (svg.match(/<title id="cp-title-avi">/g) || []).length;
+    expect(titleCount).toBe(1);
+    const styleImportCount = (svg.match(/@import url/g) || []).length;
+    expect(styleImportCount).toBe(1);
+  });
+});
+
 describe('buildTowerPaths', () => {
   it('returns correct paths for scale 1', () => {
     const paths = buildTowerPaths(15, 1);