docs: add testing guide for utility functions and coverage requirements

docs/utils-testing-guide.md

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+# Utils Testing Guide
+
+## Naming Convention & Co-location
+
+- Test files should be named **`<helperName>.test.ts`** (or `.test.tsx` for React‑related helpers).
+- Place the test file **side‑by‑side** with the helper it exercises, inside the same directory.
+
+  Example directory layout:
+  ```
+  src/utils/
+    ├─ formatNumber.utils.ts
+    └─ formatNumber.utils.test.ts   ← test file
+  ```
+
+## Running Only Util Tests
+
+The project uses **Vitest** as the test runner (configured in `vitest.config.ts`).
+
+- To run **all** tests: `pnpm test`
+- To run **only utils** tests: 
+  ```bash
+  pnpm test "src/utils/**/*.test.ts"
+  ```
+  This pattern matches every test file under `src/utils`.
+
+## Worked Example Test
+
+Below is a simple example for a pure helper `formatNumber` that formats a number with commas and two decimal places.
+
+```ts
+// src/utils/formatNumber.utils.test.ts
+import { describe, expect, it } from 'vitest';
+import { formatNumber } from './formatNumber.utils';
+
+describe('formatNumber utils', () => {
+  it('formats an integer with commas', () => {
+    expect(formatNumber(1234567)).toBe('1,234,567.00');
+  });
+
+  it('formats a floating‑point number with two decimals', () => {
+    expect(formatNumber(1234.5)).toBe('1,234.50');
+  });
+
+  it('handles negative numbers', () => {
+    expect(formatNumber(-9876.543)).toBe('-9,876.54');
+  });
+});
+```
+
+### Explanation
+- **`describe`** groups related tests under a readable heading.
+- **`it`** defines individual test cases.
+- **`expect(...).toBe(...)`** performs the assertion.
+- Because `formatNumber` is a **pure function** (no side‑effects), we can achieve **100 % branch coverage** with the three cases above (positive, decimal, negative).
+
+## Branch Coverage Expectation
+
+- **Pure helpers** (functions that depend only on their inputs) must have **100 % branch coverage**.
+- Run the coverage report with:
+  ```bash
+  pnpm test --coverage
+  ```
+- Ensure the generated `coverage` report shows `100%` for each pure helper file.
+
+---
+*This guide lives in the repository under `docs/utils-testing-guide.md` and should be referenced by contributors when adding new utility helpers.*