Optimize spawn_batch + relations

@chescock in https://github.com/bevyengine/bevy/pull/19519#discussion_r2175180498_ says:

With this change, every branch in this hook (after the `RelationshipHookMode` check) is guaranteed to run a command, right? It might make sense as a follow-up PR to wrap the entire method in `world.commands.queue(|world| {...})`. That would avoid having to look up `target_entity` twice, which would also avoid the possibility of it getting despawned in between.

Huh, and it also looks like `EntityEntryCommands::or_insert_with` just invokes the closure immediately. If we work with `world` instead of `commands` then I think we can avoid allocating a new `RelationshipTarget` if it already exists. Also, I'm going to create an issue for `or_insert_with`, because that behavior is surprising ... #19884.