[OP#73293] Creates POST /api/v3/wiki_page_links endpoint (#23529)

* Remove the `render_author` method as it is unnecessary
* Add the necessary paths, clarify the param to wiki_provider path
* Rework the representers, merge upstream changes to the create contract
* Remove unnecessary mixin and make everything read-only
* Create Endpoint for POST /api/v3/wiki_page_links
* Incorporates @Kharonus feedback in testing multiple page link creation

lib/api/utilities/endpoints/modify.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #-- copyright
 # OpenProject is an open source project management software.
 # Copyright (C) the OpenProject GmbH
@@ -31,7 +33,7 @@ module API
     module Endpoints
       class Modify < Bodied
         def default_instance_generator(model)
-          ->(_params, _current_user) do
+          lambda do |_params, _current_user|
             instance_variable_get(:"@#{model.name.demodulize.underscore}")
           end
         end
@@ -79,12 +81,13 @@ module API
             key,
             dependent_class: result.model_name.human,
             related_id: result.id,
-            # TODO: Make it more robust, as not every model has a 'name' attribute (e.g. fall back to collection index?)
-            related_subject: result.name,
+            related_subject: dependent_error_subject(result),
             error: full_message
           )
         end
 
+        def dependent_error_subject(dependent_result) = dependent_result.name
+
         def deduce_process_service
           lookup_namespaced_class("#{update_or_create}Service")
         end