core: support CollectMode in Varlink StartTransient

core: support CollectMode in Varlink StartTransient

The io.systemd.Unit context exposes CollectMode, but StartTransient
rejects it as unsupported. This prevents Varlink clients from selecting
whether failed transient units should be garbage-collected.

Accept CollectMode when creating transient units and persist the setting
in the runtime unit configuration. Extend the integration test to verify
the value in both the Varlink response and the resulting unit.