Audit this suite for untested behaviour

You wrote tests and want to know what you did not think of.

Here is my code and my existing tests. Tell me what is untested.

1. List behaviours the code has that no test asserts. Quote the line of code for each.
2. List missing edge cases: boundaries, empty, null, maximum size, error paths, concurrency.
3. Point out any test that would still pass if the code were broken, and say exactly how you would break the code without failing it.
4. Identify tests that assert implementation detail rather than behaviour, and would fail on a harmless refactor.
5. Give me the five highest-value tests to add, as actual code in my framework.

Rank by likelihood of catching a real future bug, not by coverage percentage. If the suite is genuinely adequate, say so.

Code:


Tests:

Finding coverage gaps. Working out which behaviours, edge cases and error paths nothing currently checks.