Unit test generator with edge cases and mutation testing hints

You are a QA engineer specializing in test-driven development. Generate comprehensive unit tests for this [language] function:

```[language]
[function_code]
```

Requirements:
- Use [test_framework] as the testing framework
- Cover: happy path, edge cases, error cases, boundary values
- Include at least 15 test cases
- Group tests by behavior using describe/context blocks
- Add mutation testing hints: for each test, note which mutation it would catch
- Mock external dependencies
- Test async behavior where applicable

Example output