Add spec for rescuing order saving shipment

spec/models/spree/order_spec.rb

@@ -756,6 +756,24 @@ describe Spree::Order do
 
         expect { order.next! }.to change { order.state }.from("delivery").to("payment")
       end
+
+      it "advances to complete state despite error" do
+        advance_to_delivery_state(order)
+        # advance to payment state
+        order.next!
+
+        create(:payment, order: order)
+        # https://github.com/openfoodfoundation/openfoodnetwork/issues/3924
+        observed_error = ActiveRecord::RecordNotUnique.new(
+          "PG::UniqueViolation",
+          StandardError.new
+        )
+        expect(order.shipment).to receive(:save).and_call_original
+        expect(order.shipment).to receive(:save).and_call_original
+        expect(order.shipment).to receive(:save).and_raise(observed_error)
+
+        expect { order.next! }.to change { order.state }.from("payment").to("complete")
+      end
     end
 
     context "when the order is a subscription" do