test(generateMonthlySVG): verify auto-theme CSS variables (JhaSourav07#1909)

## Description

Fixes JhaSourav07#1048

Added a test for `generateMonthlySVG` with `autoTheme: true`.

The test verifies that the generated monthly SVG contains:
- `--cp-bg`
- `--cp-accent`
- `prefers-color-scheme: dark`

## 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.
- [ ] 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 community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/svg/generator.test.ts

@@ -965,7 +965,6 @@ describe('generateMonthlySVG', () => {
     expect(svg).toContain('width="400"');
     expect(svg).toContain('height="200"');
   });
-
   it('includes prefers-reduced-motion media query in static monthly SVG output', () => {
     const svg = generateMonthlySVG(mockMonthlyStats, {
       user: 'octocat',
@@ -987,6 +986,17 @@ describe('generateMonthlySVG', () => {
     expect(svg).toContain('transition: none !important');
   });
 
+  it('includes CSS variables in auto-theme monthly SVG', () => {
+    const svg = generateMonthlySVG(mockMonthlyStats, {
+      user: 'octocat',
+      autoTheme: true,
+    } as unknown as BadgeParams);
+
+    expect(svg).toContain('--cp-bg');
+    expect(svg).toContain('--cp-accent');
+    expect(svg).toContain('prefers-color-scheme: dark');
+  });
+
   it('supports dynamic Google Fonts for non-predefined fonts in monthly auto-theme mode', () => {
     const svg = generateMonthlySVG(mockMonthlyStats, {
       user: 'octocat',