EntityWorldMut methods do not automatically overwrite Relationship components

[urben1680]

Objective

Some methods and commands carelessly overwrite Relationship components. This may overwrite additional data stored at them which is undesired.

Part of #19589

Solution

A new private method will be used instead of insert: modify_or_insert_relation_with_relationship_hook_mode.

This method behaves different to insert if Relationship is a larger type than Entity and already contains this component. It will then use the modify_component API and a new Relationship::set_risky method to set the related entity, keeping all other data untouched.

For the replace_related(_with_difference) methods this also required a InsertHookMode parameter for efficient modifications of multiple children. The changes here are limited to the non-public methods.

I would appreciate feedback if this is all good.

Testing

Added tests of all methods that previously could reset Relationship data.

[alice-i-cecile]

Okay, this fix makes sense to me, the code is good, the migration guide is good and there are tests. I have a couple small suggestions, but nothing blocking.

[copygirl]

Only got a final nitpick, otherwise I couldn't see anything that stood out to me.

(Got pleasantly surprised I was asked to review, hope I'm doing this properly.)

[alice-i-cecile]

(Got pleasantly surprised I was asked to review, hope I'm doing this properly.)

It's much appreciated!