Thorough Unit Test Writer

Write unit tests for the code below using {{test_framework}} in {{language}}.

Code under test:
{{code}}

Cover: the happy path; boundary values (empty, one, many, max, negative, unicode, very large); invalid input and the exact error behaviour; idempotency or ordering where relevant; time, randomness and external calls mocked at the boundary; and any bug-prone branches you notice. Name tests as behaviour sentences ('returns an empty list when no items match'). Use arrange/act/assert, no logic inside tests, and no assertions on private implementation. After the tests, list behaviours you could not test with the information given and what you would need.