test(svg): add accessibility coverage (JhaSourav07#3145)

## Description

Fixes JhaSourav07#2886 

Added isolated unit tests for `lib/svg/constants.ts` focusing on
accessibility-related configuration stability and SVG rendering
prerequisites.

### Changes
- Added `lib/svg/constants.accessibility.test.ts`
- Verified SVG dimension constants contain valid positive values
- Tested font mapping definitions used for readable SVG text rendering
- Confirmed contribution milestone values remain valid and positive
- Verified streak milestone values remain valid and positive
- Tested milestone collections maintain ascending ordering for
predictable presentation and accessibility-friendly consumption
- Improved coverage around accessibility-supporting SVG configuration
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.accessibility.test.ts

@@ -0,0 +1,50 @@
+// lib/svg/constants.accessibility.test.ts
+
+import { describe, expect, it } from 'vitest';
+import {
+  SVG_WIDTH,
+  SVG_HEIGHT,
+  FONT_MAP,
+  CONTRIBUTION_MILESTONES,
+  STREAK_MILESTONES,
+} from './constants';
+
+describe('SVG Constants Accessibility', () => {
+  it('provides valid SVG dimensions for accessible rendering', () => {
+    expect(SVG_WIDTH).toBeGreaterThan(0);
+    expect(SVG_HEIGHT).toBeGreaterThan(0);
+  });
+
+  it('exposes readable font mappings for SVG text content', () => {
+    expect(FONT_MAP).toHaveProperty('jetbrains');
+    expect(FONT_MAP).toHaveProperty('fira');
+    expect(FONT_MAP).toHaveProperty('roboto');
+
+    Object.values(FONT_MAP).forEach((font) => {
+      expect(typeof font).toBe('string');
+      expect(font.length).toBeGreaterThan(0);
+    });
+  });
+
+  it('defines contribution milestone values suitable for accessible labels', () => {
+    expect(CONTRIBUTION_MILESTONES.length).toBeGreaterThan(0);
+
+    CONTRIBUTION_MILESTONES.forEach((value) => {
+      expect(value).toBeGreaterThan(0);
+    });
+  });
+
+  it('defines streak milestone values suitable for accessible labels', () => {
+    expect(STREAK_MILESTONES.length).toBeGreaterThan(0);
+
+    STREAK_MILESTONES.forEach((value) => {
+      expect(value).toBeGreaterThan(0);
+    });
+  });
+
+  it('maintains ascending milestone ordering for predictable narration', () => {
+    expect(CONTRIBUTION_MILESTONES).toEqual([...CONTRIBUTION_MILESTONES].sort((a, b) => a - b));
+
+    expect(STREAK_MILESTONES).toEqual([...STREAK_MILESTONES].sort((a, b) => a - b));
+  });
+});