Validate serials in xdg_activation before focusing

Discussion link

https://github.com/hyprwm/Hyprland/discussions/13999

Description

Currently, xdg_activation doesn't check if the provided serial is valid, recent, or anything, allowing apps to just randomly steal focus.

It should probably be allowed for that to be the case should the user configure it that way, but by default we should reject bad serials.