Request carrier launch

This is what makes an agent reach non-test devices. Reachability is carrier-scoped, so an agent launched on two carriers is invisible on a third.

carriers replaces the whole launch set at Google rather than adding to it — previously requested carriers are re-sent automatically, because omitting them would silently drop those launches while the request still succeeded.

Verification must have been submitted first, but it only completes during launch approval: requiring verified here would deadlock, since verification never starts until a launch exists.