Generate Tests for a Module

Add tests for this module using the test stack already in the repo.

Module / files: {{module}}
Behaviour that must not regress:
{{behaviour}}
Known edge cases:
{{edges}}

Rules:
- Mirror existing test file placement and naming
- Prefer behaviour over implementation details
- Include the unhappy paths in {{edges}}
- Do not mock so much that the test asserts nothing
- If a case needs a fixture, create a small one next to existing fixtures

End with: how to run only these tests, and any behaviour you could not cover because the code is untestable — with a minimal seam if needed.