[server] Send text input leave if focused surface gets unbound
Summary: As 98e5d269a110dca9cae1e4ed2bb3051f02a69bff just for text input.
Test Plan: Adjusted test passes, fails without adjustment
Reviewers: Plasma, Frameworks
Subscribers: plasma-devel
Tags: Plasma on Wayland, Frameworks
Differential Revision: https://phabricator.kde.org/D6786