Fix replace_related allowing duplicate entities

[SkiFire13]

Objective

• Make replace_related not insert duplicate Entity if the given collections contains them.
• Helps with UB found in EntityWorldMut::replace_children in multi-threaded with pure safe code #25349

Solution

• Collect into a EntityHashSet before replacing the underlying relationship target collection
• This is NOT supposed to prevent Children or other relationship targets from containing duplicates, however it makes this less likely to happen by mistake.

Testing

• Added a regression test