Ensure that deleting an offsite link from a standard edition only removes the association, not the offsite link itself

In the existing implementation, deleting an offsite link from a standard edition would delete the offsite link record itself,
which would affect other editions that are associated with the same offsite link.
I've made sure only the association between the edition and the offsite link is removed,
while retaining the offsite link record or any other editions that may be using it.
This follows the established design from #11001

Author: eYinka

app/controllers/admin/offsite_links_controller.rb

@@ -30,7 +30,11 @@ def update
   def confirm_destroy; end
 
   def destroy
-    @offsite_link.destroy!
+    if @parent.is_a?(Edition)
+      @parent.offsite_link_parents.find_by(offsite_link: @offsite_link)&.destroy!
+    else
+      @offsite_link.destroy!
+    end
     flash[:notice] = "#{@offsite_link.title} has been deleted"
     redirect_to offsite_links_path
   end

test/functional/admin/offsite_links_controller_test.rb

@@ -154,10 +154,12 @@ class Admin::OffsiteLinksControllerTest < ActionController::TestCase
     assert_select "p", text: "Are you sure you want to delete \"#{offsite_link.title}\" from \"#{edition.title}\"?"
   end
 
-  view_test "DELETE :destroy removes offsite link for standard edition parent" do
+  view_test "DELETE :destroy removes the association between an offsite link and an edition but does not delete the offsite link" do
     edition = create(:standard_edition, :published)
     offsite_link = create(:offsite_link, editions: [edition])
-    assert_difference("OffsiteLink.count", -1) do
+
+    assert_equal [offsite_link], edition.offsite_links
+    assert_no_difference("OffsiteLink.count") do
       delete :destroy, params: {
         standard_edition_id: edition, id: offsite_link.id
       }