Responses that aren't `()` should maybe be `#[must_use]`?

Here's a situation that I found myself in:

Before

• Crucible used to return Result<HttpResponseOk<()>, HttpError> from a server endpoint (job_result_ok in https://github.com/oxidecomputer/crucible/pull/654/files )
• This generated a client API which returned a Result<(), _> type. The callsites all used ? to check the error, but didn't bother assigning the Ok type to a variable, because why bother?

After

• After that PR (crucible#654), the endpoint returned a structure -- JobResultOkResponse
• However, all the callsites continued to ignore that result type, with no warning.

I think it's fair to say this isn't a "bug", but it's a pain-in-the-butt -- it requires all the callsites to be manually updated to not drop useful information on the floor. In our case, the #[must_use] declaration would have made it very clear what would need to change in the caller, and would be easy to dismiss if it wasn't relevant.

Start by comparing the generated client behavior described for Crucible’s job_result_ok endpoint before and after crucible#654, then trace how Progenitor represents non-() response types. Check the relevant generator tests and add coverage showing that ignored useful responses produce the intended #[must_use] guidance.