Work item introduced by #4856.

For now, as long as conpty can parse AppKeys and some mouse events, we don't need to turn input passthrough off. Since the VT is only passed through if the app is ready for it (denoted by ENABLE_VIRTUAL_TERMINAL_INPUT), we stop the VT Input at InputStateMachineEngine. So it should be OK. Not ideal, but it works.

when we fix this, revert or remove the fix for #4911