test(svg): add mouse interactivity coverage (JhaSourav07#3143)

## Description

Fixes JhaSourav07#2887 

Added isolated unit tests for `lib/svg/constants.ts` focusing on
configuration stability, milestone integrity, and SVG rendering
prerequisites.

### Changes
- Added `lib/svg/constants.mouse-interactivity.test.ts`
- Verified SVG dimension constants contain valid positive values
- Tested font mapping definitions used throughout SVG rendering
- Confirmed contribution milestone values remain ordered and valid
- Verified streak milestone values remain ordered and valid
- Tested milestone collections contain positive thresholds suitable for
downstream interactive features
- Improved coverage around SVG configuration constants and
rendering-related data structures

## 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 changes)

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally.
- [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 rendering changes made).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/svg/constants.mouse-interactivity.test.ts

@@ -0,0 +1,43 @@
+// lib/svg/constants.mouse-interactivity.test.ts
+
+import { describe, expect, it } from 'vitest';
+import {
+  SVG_WIDTH,
+  SVG_HEIGHT,
+  FONT_MAP,
+  CONTRIBUTION_MILESTONES,
+  STREAK_MILESTONES,
+} from './constants';
+
+describe('SVG Constants Mouse Interactivity', () => {
+  it('provides valid SVG dimensions for interactive layouts', () => {
+    expect(SVG_WIDTH).toBeGreaterThan(0);
+    expect(SVG_HEIGHT).toBeGreaterThan(0);
+  });
+
+  it('exposes font mappings used by interactive SVG elements', () => {
+    expect(FONT_MAP).toHaveProperty('jetbrains');
+    expect(FONT_MAP).toHaveProperty('fira');
+    expect(FONT_MAP).toHaveProperty('roboto');
+  });
+
+  it('defines contribution milestone values in ascending order', () => {
+    const sorted = [...CONTRIBUTION_MILESTONES].sort((a, b) => a - b);
+
+    expect(CONTRIBUTION_MILESTONES).toEqual(sorted);
+  });
+
+  it('defines streak milestone values in ascending order', () => {
+    const sorted = [...STREAK_MILESTONES].sort((a, b) => a - b);
+
+    expect(STREAK_MILESTONES).toEqual(sorted);
+  });
+
+  it('contains milestone values suitable for interactive overlays', () => {
+    expect(CONTRIBUTION_MILESTONES.length).toBeGreaterThan(0);
+    expect(STREAK_MILESTONES.length).toBeGreaterThan(0);
+
+    expect(CONTRIBUTION_MILESTONES.every((v) => v > 0)).toBe(true);
+    expect(STREAK_MILESTONES.every((v) => v > 0)).toBe(true);
+  });
+});