Fix extension cords falling off when items are turned on

[Kamayana]

Summary

Bugfixes "Fix extension cords falling off when items are turned on"

Purpose of change

Fixes #74246.

Describe the solution

When writing #70475, I incorrectly made item_contents::combine rely on pocket.saved_type() even when it's called during item conversion. I didn't realize that saved_type it only ever useful during deserialization, otherwise it's always LAST. The function already used the convert parameter for alternative code that checked against pocket.get_pocket_data()->type instead, which is accurate past loading, so simply applying that to the problem code fixes it.

The other part of the fixed issue, the cable "duplicating", was because the item wasn't updating its cable length after conversion made the extension cord fall out. The extension cord wasn't duplicating, it was more like there was a phantom extension cord left behind that kept the item connected at its longer length. Calling update_link_traits() during item conversion fixes this.

Describe alternatives you've considered

Testing

Followed the steps in the issue, no longer reproduces. Followed the steps with only the second commit applied, the lamp immediately updates the cable length and breaks the connection, as intended.

Also spawned and connected some extension cords together and to a battery to make sure nothing unintentionally broke.

Additional context