Servlet extension returns inconsistent response code when multipart request body is too large

### Bug description

An oddity discovered when working on https://github.com/apache/camel-quarkus/issues/5326.

If you repeatedly cURL `/my-named-folder/multipart` with a request body larger than the allowed limit, the response code returned is not consistent. Sometimes you get 413 and sometimes it's 500 due to `UT000139 Exchange already complete`.