Write Unit Tests

When writing unit tests for a function or module:

1. Identify the public API — the functions and methods that consumers call.
2. For each public function, list the behaviors to test: normal cases, edge cases, error cases.
3. Write a test for the most common usage first (the happy path).
4. Add edge case tests: empty input, null, zero, negative numbers, very large inputs, special characters.
5. Add error case tests: invalid input, missing required fields, network failures.
6. Name each test to describe the expected behavior, not the implementation.
7. Use arrange-act-assert structure: set up data, call the function, check the result.
8. Run the tests to confirm they pass. Then break the implementation to confirm they catch real failures.