Add a suite of functional tests

There should be one test per endpoint, and you should be able to run the same set of tests against every variation using `[Theory]` and `InlineData` or similar parameterized tests.

Start by locating the project's endpoint implementations and existing test project or test conventions; the issue does not name specific files. Run the current test suite, then add one functional test per endpoint and parameterize the same tests across each variation with Theory and InlineData or similar; done means every endpoint and variation is covered.

Written by the indexing model from the issue text.