Rework one-to-one relationships to remove existing instead of panicking

[muddxyii]

Objective

• Issue: Rework one-to-one relations to remove the original relationship rather than panicking #18847
• Resolves panic when establishing a one-to-one relationship to an entity that already has an existing relationship from another entity.

Solution

• Modified the on_insert hook in the Relationship trait to detect one-to-one relationships (where the target collection type is Entity) and automatically remove any existing relationship before establishing the new one.
• Uses runtime type checking with TypeId to identify one-to-one vs one-to-many relationships.
• Safely removes the existing source relationship using try_remove() to handle edge cases gracefully.

Testing

• Removed panic assertions from Entity::add() and Entity::extend_from_iter() methods that would previously crash when attempting to establish overlapping one-to-one relationships
• Modified existing test one_to_one_relationship_shared_target by removing #[should_panic] and adding assertions to verify:
    - Original relationship is properly removed
    - New relationship is correctly established
    - Target entity points to the new source

[chescock]

Does this have a race condition if we do a batch insert, similar to the one #19519 fixed? If we insert multiple one-to-one relationship sources without flushing, this will remove any previous source from the target, but I think later sources in the batch will overwrite earlier ones without removing.

This might be easier to fix if we merge #19932 first and do this whole check inside the command.

[chescock]

Looks good!

I'm still worried about reintroducing a race condition in batch inserts, especially since we just fixed one there, but we can probably fix that in a follow up after #19932.