Fix #28639. Use api_v3_path helpers for linking work package's available relation candidates

lib/api/v3/utilities/path_helper.rb

@@ -406,6 +406,10 @@ module API
             "#{work_package_relations(work_package_id)}/#{id}"
           end
 
+          def self.work_package_available_relation_candidates(id)
+            "#{work_package(id)}/available_relation_candidates"
+          end
+
           def self.work_package_revisions(id)
             "#{work_package(id)}/revisions"
           end

lib/api/v3/work_packages/work_package_representer.rb

@@ -138,7 +138,7 @@ module API
           next if represented.new_record?
 
           {
-            href: "/api/v3/work_packages/#{represented.id}/available_relation_candidates",
+            href: api_v3_paths.work_package_available_relation_candidates(represented.id),
             title: "Potential work packages to relate to"
           }
         end