Rescue checkout on shipping id conflict

Helps to investigate https://github.com/openfoodfoundation/openfoodnetwork/issues/3924

A better solution would be to move the update of shipments out of the
after_save callback and deal with it in controllers. Unfortunately,
that's a big and tricky task.

Since this exception is causing a lot of pain for some Australian
farmers, I introduced more logging here to understand the problem
better. The issue was observed in OFN v1 and may disappear with v2. But
we don't know that and should monitor it.

app/models/spree/order_decorator.rb

@@ -166,7 +166,21 @@ Spree::Order.class_eval do
     shipments.each do |shipment|
       next if shipment.shipped?
       update_adjustment! shipment.adjustment if shipment.adjustment
-      shipment.save # updates included tax
+      begin
+        shipment.save # updates included tax
+      rescue ActiveRecord::RecordNotUnique => error
+        # This error was seen in production on `shipment.save` above.
+        # It caused lost payments and duplicate payments due to database rollbacks.
+        # While we don't understand the cause of this error yet, we rescue here
+        # because an outdated shipping fee is not as bad as a lost payment.
+        # And the shipping fee is already up-to-date when this error occurs.
+        # https://github.com/openfoodfoundation/openfoodnetwork/issues/3924
+        Bugsnag.notify(error) do |report|
+          report.add_tab(:order, attributes)
+          report.add_tab(:shipment, shipment.attributes)
+          report.add_tab(:shipment_in_db, Spree::Shipment.find_by_id(shipment.id).attributes)
+        end
+      end
     end
   end