test(validations): add tests for scale and size fallback behavior in streakParamsSchema (JhaSourav07#881)

## Description
Fixes JhaSourav07#661

## Pillar
- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
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- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
N/A — test-only PR, no SVG output changes

## Checklist before requesting a review:
- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally (21 tests pass via npx vitest
run lib/validations.test.ts)
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors.
- [x] My commits follow the Conventional Commits format.
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parameter.
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Author: JhaSourav07

lib/validations.test.ts

@@ -102,3 +102,62 @@ describe('streakParamsSchema user validation', () => {
     }
   });
 });
+
+// Helper — parse only the fields we care about, supplying the required `user` field.
+function parse(params: Record<string, string>) {
+  return streakParamsSchema.parse({ user: 'octocat', ...params });
+}
+
+describe('streakParamsSchema — scale fallback behavior', () => {
+  // z.enum(['linear', 'log']).catch('linear') — unknown values silently fall
+  // back to 'linear' instead of throwing a validation error.
+
+  it('accepts "log" as a valid scale value', () => {
+    expect(parse({ scale: 'log' }).scale).toBe('log');
+  });
+
+  it('accepts "linear" as a valid scale value', () => {
+    expect(parse({ scale: 'linear' }).scale).toBe('linear');
+  });
+
+  it('falls back to "linear" for unknown scale value', () => {
+    expect(parse({ scale: 'exponential' }).scale).toBe('linear');
+  });
+
+  it('falls back to "linear" for empty string', () => {
+    expect(parse({ scale: '' }).scale).toBe('linear');
+  });
+
+  it('defaults to "linear" when scale is omitted', () => {
+    expect(parse({}).scale).toBe('linear');
+  });
+});
+
+describe('streakParamsSchema — size fallback behavior', () => {
+  // z.enum(['small', 'medium', 'large']).catch('medium') — unknown values
+  // silently fall back to 'medium' to preserve badge rendering.
+
+  it('accepts "small" as a valid size value', () => {
+    expect(parse({ size: 'small' }).size).toBe('small');
+  });
+
+  it('accepts "medium" as a valid size value', () => {
+    expect(parse({ size: 'medium' }).size).toBe('medium');
+  });
+
+  it('accepts "large" as a valid size value', () => {
+    expect(parse({ size: 'large' }).size).toBe('large');
+  });
+
+  it('falls back to "medium" for unknown size value', () => {
+    expect(parse({ size: 'giant' }).size).toBe('medium');
+  });
+
+  it('defaults to "medium" when size is omitted', () => {
+    expect(parse({}).size).toBe('medium');
+  });
+
+  it('falls back to "medium" for empty string', () => {
+    expect(parse({ size: '' }).size).toBe('medium');
+  });
+});