Move persistence of relations from repository into notification handlers

[AndyButland]

Description

See #20079 (review) for the background to raising this PR, which relate to optimising and making more reliable save operations to support Deploy operations.

Also #20094.

I've moved the persistence of relations from the repositories into a notification handler.

In testing I can see it working as it did before.

Referencing some discussion from when this was first implemented around whether this should be carried out in an event (at the time, as it was Umbraco 8), or in the repository itself:
https://github.com/umbraco/Umbraco-CMS/pull/6843/files#r338926734

Testing

Via a database query, find the references for a given parent item - e.g. a document, that has pickers for documents, media or members; or any other property editor that holds references to other nodes:

select * from umbracoRelation where ParentId = {id}

Experiment with adding and removing related items, saving and publishing, and verifying that the related records are as expected.

[Copilot]

Pull Request Overview

This PR refactors the persistence of relations from being handled directly in repositories to using notification handlers, improving the architecture and supporting Deploy operations optimization.

Key changes:

• Removes PersistRelations calls from repository persistence methods
• Creates a new ContentRelationsUpdate notification handler to manage relation persistence
• Registers the notification handler for content saved/published events

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
MemberRepository.cs	Removes PersistRelations calls from new and updated item persistence methods
MediaRepository.cs	Removes PersistRelations calls from new and updated item persistence methods
DocumentRepository.cs	Removes PersistRelations calls from new and updated item persistence methods
ContentRepositoryBase.cs	Marks PersistRelations method as obsolete and converts to no-op with warning log
ContentRelationsUpdate.cs	New notification handler implementing relation persistence logic for all content types
UmbracoBuilder.CoreServices.cs	Registers the new notification handler for content saved/published events