Management API: Return not found from request for content references when entity does not exist (closes #20997)

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Addresses: #20997

Description

Between 16 and 17 it is reported that these reference endpoints would return all items instead of an empty collection when the provided ID was not recognised. It seems better really that these would return "not found", which is what I've done in this PR.

Testing

Using the management API, make requests to e.g. umbraco/management/api/v1/media/{id}/referenced-descendants and verify that providing an ID that doesn't exist, or isn't of the right type, returns a 404 response.

[Copilot]

Pull request overview

This PR fixes a regression in the Management API where reference endpoints (ReferencedBy and ReferencedDescendants) would return all items instead of a 404 Not Found response when an invalid entity ID was provided. The change adds entity existence validation using IEntityService before processing reference queries.

Key changes:

• Added 404 Not Found validation for all reference endpoints across Document, Media, and Member controllers
• Introduced IEntityService dependency injection with backward-compatible obsolete constructors
• Added ProducesResponseType attributes documenting the new 404 responses

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
ReferencedDescendantsMemberController.cs	Added entity validation to return 404 for non-existent member IDs
ReferencedByMemberController.cs	Added entity validation to return 404 for non-existent member IDs
ReferencedDescendantsMediaController.cs	Added entity validation to return 404 for non-existent media IDs
ReferencedByMediaController.cs	Added entity validation to return 404 for non-existent media IDs
ReferencedDescendantsDocumentController.cs	Added entity validation to return 404 for non-existent document IDs
ReferencedByDocumentController.cs	Added entity validation to return 404 for non-existent document IDs