#[connect_impl] — inserts req.validate()? at the top of every Connect
service handler method in an impl block whose request parameter is an
OwnedView<_>. Single-site safety net: add it once to the service impl
and every present-and-future handler is validated on entry.
Non-handler async fns inside the same impl block are left alone
(they lack an OwnedView<_> parameter, so the macro skips them).