OData clients can trigger various 501 response codes

The following OData errors look like they should be 4xx:

https://github.com/getodk/central-backend/blob/8b9db34bd91d03ca7d2f7719acc5363b1871ea03/lib/util/problem.js#L239-L242

* 5xx errors are for server-side issues
* 501 is for unsupported HTTP methods:

> This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.
> —_https://www.rfc-editor.org/rfc/rfc9110#status.501_

Start at lib/util/problem.js lines 239-242 and trace the OData errors that produce 501 responses. Compare each condition with RFC 9110 and the project's existing response handling; done means these client-caused OData errors return appropriate 4xx codes rather than 501.

Written by the indexing model from the issue text.