run: pass the pty slave fd to transient service

run: pass the pty slave fd to transient service

The rationale is similar to 40e1f4ea7458a0a80eaf1ef356e52bfe0835412e.

Currently, we only pass TTYPath=/dev/pts/... to
the transient service spawned by systemd-run.
This is a bit problematic though, when ExecStartPre=
or ExecStopPost= is used. Since when these control
processes get to run, the main process is not yet
started/has already exited, hence the slave suffers
from the same vhangup problem as the mentioned commit.

By passing the slave fd in, the service manager will
hold the fd open as long as the service is alive.

Fixes #32916