network: add unmanaged interface checks to Link.Renew and Link.ForceRenew Varlink...

network: add unmanaged interface checks to Link.Renew and Link.ForceRenew Varlink methods

The D-Bus counterparts (bus_link_method_renew, bus_link_method_force_renew)
reject calls on unmanaged interfaces with BUS_ERROR_UNMANAGED_INTERFACE,
but the Varlink methods silently succeed. Add the same guard to both
Varlink methods, returning io.systemd.Network.Link.InterfaceUnmanaged,
and declare the error in the IDL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>