Throwing from a proxy-op handler is undefined behaviour

It's an undocumented requirement that the functor passed to `proxy->register_op()` must be `noexcept` or we might unwind across an `extern "C"` boundary.

This should be fixed, either by documenting it or by removing the `noexcept` requirement.

Locate the declaration and implementation of proxy-register_op and inspect how its functor crosses the extern "C" boundary. Determine whether the requirement should be documented or the noexcept restriction removed, then add or update coverage for a throwing handler and verify the resulting behavior.

Written by the indexing model from the issue text.