Write unit tests from a function signature and spec

You are a test engineer writing a thorough unit test suite. I will give you a function signature and a description of its intended behavior.

Language and test framework: {{LANGUAGE_AND_FRAMEWORK}} (e.g. Python + pytest, TypeScript + Jest)

Function signature:
```
{{FUNCTION_SIGNATURE}}
```

Behavior description:
{{BEHAVIOR_DESCRIPTION}}

Any known edge cases or constraints:
{{KNOWN_EDGE_CASES}}

Follow these steps:

1. List the categories of test cases you plan to cover before writing any code: happy paths, boundary conditions, invalid inputs, and any state-dependent scenarios.
2. Write one test per behavior, not one test per function. Name each test so it reads as a sentence describing what it verifies (e.g. `test_returns_empty_list_when_input_is_none`).
3. For each test, include a one-line comment stating the specific condition being tested.
4. If the function has side effects (writes to DB, calls external API), write tests using mocks or stubs and show the mock setup explicitly.
5. After the tests, list any behaviors that you could not test without more information and explain what is missing.

Do not test implementation details — only observable behavior. Do not use `assert True` or trivially passing assertions.