test: add a bunch of tests

[MarshallOfSound]

At one point this module had really good test coverage, then we added a bunch of stuff to it 😆 These are mostly claude generated test cases but I've gone through them and they all seem like in/out samples

[socket-security[bot]]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​vitest/​coverage-v8@​3.0.5

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[MarshallOfSound]

Some of these tests have expect calls inside conditional branches, which is a code smell. I'm guessing that was Claude trying to make the code pass type checking, but it can lead to those expect calls never running and the tests passing if something subtle changes. Those can be refactored using assert to both pass type checking and not be in a conditional branch that could get skipped.

This is a good call out that I missed 🙇 I updated all these to use type assertions instead as these are frequently code patterns of "assert foo -> assert foo.bar" where typescript doesn't understand vitests "expect" syntax is actually a typecheck.

Things looking a bit better now and typecheck now passing