* Fix subscription custom configs ignoring custom config socks port
Full Xray/sing-box configs imported from a subscription no longer got the
subscription's "Custom config socks port" (SubItem.PreSocksPort) after
eff5845 and 9638b4c. 7.24.4 and earlier copied it onto every imported
config, and the Clash, Hysteria2 and raw-file paths still do.
Without it, GetPreSocksItem builds no pre-socks core for these profiles,
so they cannot be used in TUN mode. Setting the port on the profile is
not a workaround, because every subscription update re-creates the
profiles.
* Add regression test for subscription custom config socks port
