Fix tow cables crashing and not keeping connections properly

[Kamayana]

Summary

Bugfixes "Fix tow cables crashing and not keeping connections properly"

Purpose of change

Fixes #71412. Also fixes tow cables being fully disconnected when you only detach one end - the other end now stays connected, as with other cable links.

Describe the solution

Adds a check for a valid vehicle part, rather than naïvely assuming it'll always be valid. Adjusts the order invalidate_towing handles things so it removes parts and then makes a connection, since tow cables can't be attached to vehicles that already have one.

Describe alternatives you've considered

Tow cables still have a bunch of weird jank to them, but here I just wanted to fix them back to the workable state they were in before.

Testing

Repeated the steps of the linked issue, no problem. Attached a tow cable to two cars in both directions, both worked, cars tow properly. Tried connecting both cable ends to same car, fail as intended. While towing car, map edit a stone wall in between to make sure the cable snaps off as it should; it does.

Additional context