#[connect_impl] — validates the request view at the top of every Connect
service handler method in an impl block whose request parameter is an
OwnedView<_> or connectrpc::ServiceRequest<'_, _>. Single-site safety
net: add it once to the service impl and every present-and-future handler is
validated on entry.
Validation borrows OwnedView::reborrow() or ServiceRequest::view() and
requires Validate on the generated view type. It does not convert the
request to an owned message.
Non-handler async fns inside the same impl block are left alone
(they lack a recognized request parameter, so the macro skips them).