test: validate generated SVG output with DOMParser (JhaSourav07#2236)

## Description

Fixes JhaSourav07#392 

Added SVG validation coverage to 'lib/svg/generator.test.ts' by
introducing an 'assertValidSVG()' helper that parses generated SVG
output with 'DOMParser' and verifies that no parser errors are present.

### Changes made

1. Added 'assertValidSVG(svgString: string)' helper
2. Parses SVG output using 'DOMParser' with 'image/svg+xml'
3. Verifies generated SVGs are well-formed XML/SVG documents
4. Added validation checks to 5 existing SVG generation test cases
5. Preserved all existing test assertions

## Pillar

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

## Visual Preview

N/A — test-only change, no visual SVG output was modified.

## 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=Harshitbhardwaj468').
* [x] I have run 'npm run format' and 'npm run lint' locally and
resolved all errors introduced by this change.
* [x] I have run 'npm run test' and all tests pass locally.
* [ ] I have run 'npm run test:coverage' and branch coverage is at or
above 70%.
* [x] My commits follow the Conventional Commits format.
* [ ] 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 production SVG output was modified).
* [x] (Recommended) I joined the CommitPulse Discord server for faster
collaboration, mentorship, and PR support.

### Verification

npm run test

Result:

No test failures introduced by this change

Author: JhaSourav07

lib/svg/generator.test.ts

@@ -16,6 +16,14 @@ import type { BadgeParams, ContributionCalendar, StreakStats, MonthlyStats } fro
 import { hexColor } from './sanitizer';
 import { themes } from './themes';
 
+function assertValidSVG(svgString: string): void {
+  const doc = new DOMParser().parseFromString(svgString, 'image/svg+xml');
+
+  const parserError = doc.querySelector('parsererror');
+
+  expect(parserError).toBeNull();
+}
+
 describe('generateSVG', () => {
   const mockStats: StreakStats = {
     currentStreak: 5,
@@ -50,6 +58,8 @@ describe('generateSVG', () => {
       mockCalendar
     );
 
+    assertValidSVG(svg);
+
     expect(svg).not.toContain('CURRENT_STREAK');
     expect(svg).not.toContain('ANNUAL_SYNC_TOTAL');
     expect(svg).not.toContain('PEAK_STREAK');
@@ -70,6 +80,8 @@ describe('generateSVG', () => {
       mockCalendar
     );
 
+    assertValidSVG(svg);
+
     expect(svg).toContain('CURRENT_STREAK');
     expect(svg).toContain('ANNUAL_SYNC_TOTAL');
     expect(svg).toContain('PEAK_STREAK');
@@ -78,6 +90,8 @@ describe('generateSVG', () => {
   it('uses default typography when no font is passed', () => {
     const svg = generateSVG(mockStats, { user: 'avi' } as unknown as BadgeParams, mockCalendar);
 
+    assertValidSVG(svg);
+
     expect(svg).toContain('Syncopate');
     expect(svg).toContain('Space Grotesk');
   });
@@ -89,6 +103,8 @@ describe('generateSVG', () => {
       mockCalendar
     );
 
+    assertValidSVG(svg);
+
     expect(svg).toContain('JetBrains Mono');
   });
 
@@ -99,6 +115,8 @@ describe('generateSVG', () => {
       mockCalendar
     );
 
+    assertValidSVG(svg);
+
     expect(svg).toContain('rx="0"');
   });