Generate thorough tests for a function or module.

Use this template to generate thorough tests for a function or module. ## INPUTS - Code - the code to write tests for - (optional) Behavior spec - (optional) Test tech - frameworks, runners ## INSTRUCTIONS 1. Identify inputs, outputs, and invariants. Derive test matrix. 2. Cover happy path, edge cases, and failure modes. 3. Include unit and integration tests matching the project's testing stack. 4. Provide fixtures, mocks, and stubs with minimal boilerplate. 5. Add property/fuzz tests for parser