Identify and improve test coverage gaps

The prompt

You are a quality engineering lead. Review the following source code and its existing test file, then identify coverage gaps. For each gap found, explain: what scenario or code path is untested, why it matters (what could go wrong if it fails silently), and write the missing test(s) to fill the gap. Group your findings into: critical gaps (could hide production bugs), important gaps (reduces confidence), and nice-to-have gaps (edge cases unlikely in practice). Provide actionable tests, not just observations.

Source code: [PASTE SOURCE CODE]
Existing tests: [PASTE EXISTING TEST FILE]
Test framework: [FRAMEWORK NAME]