refactor: move unit price formatting logic out of model into helper

Solves Psych::DisallowedClass errors
Example: https://github.com/openfoodfoundation/openfoodnetwork/actions/runs/14739687958/job/41374343627?pr=13232

unit_price returns object with amount and unit, as mentioned in https://github.com/openfoodfoundation/openfoodnetwork/pull/6905\#discussion_r578401368

app/helpers/spree/orders_helper.rb

@@ -45,5 +45,9 @@ module Spree
         shop: current_distributor.name,
         oc_close: l(current_order_cycle.orders_close_at, format: "%A, %b %d, %Y @ %H:%M"))
     end
+
+    def format_unit_price(unit_price)
+      "#{Spree::Money.new(unit_price[:amount]).to_html} / #{unit_price[:unit]}"
+    end
   end
 end

app/models/invoice/data_presenter/line_item.rb

@@ -4,7 +4,7 @@ class Invoice
   class DataPresenter
     class LineItem < Invoice::DataPresenter::Base
       attributes :id, :added_tax, :currency, :included_tax, :price_with_adjustments, :quantity,
-                 :variant_id, :unit_price_price_and_unit, :unit_presentation,
+                 :variant_id, :unit_price, :unit_presentation,
                  :enterprise_fee_additional_tax, :enterprise_fee_included_tax
       attributes_with_presenter :variant
       invoice_generation_attributes :added_tax, :included_tax, :price_with_adjustments,

app/models/spree/line_item.rb

@@ -234,10 +234,13 @@ module Spree
       final_weight_volume / quantity
     end
 
-    def unit_price_price_and_unit
-      unit_price = UnitPrice.new(variant)
-      Spree::Money.new(price_with_adjustments / unit_price.denominator).to_html +
-        " / ".html_safe + unit_price.unit
+    def unit_price
+      unit = UnitPrice.new(variant).unit
+      amount = price_with_adjustments / UnitPrice.new(variant).denominator
+      {
+        amount:,
+        unit:,
+      }
     end
 
     def scoper

app/serializers/invoice/line_item_serializer.rb

@@ -3,7 +3,7 @@
 class Invoice
   class LineItemSerializer < ActiveModel::Serializer
     attributes :id, :added_tax, :currency, :included_tax, :price_with_adjustments, :quantity,
-               :variant_id, :unit_price_price_and_unit, :unit_presentation,
+               :variant_id, :unit_price, :unit_presentation,
                :enterprise_fee_additional_tax, :enterprise_fee_included_tax
     has_one :variant, serializer: Invoice::VariantSerializer
 

app/views/spree/admin/orders/_invoice/_line_item_name.html.haml

@@ -2,5 +2,6 @@
   = line_item.variant.product.name
   - unless line_item.variant.product.name.include? line_item.name_to_display
     %span= "- #{line_item.name_to_display}"
-- if line_item.unit_price_price_and_unit
-  = raw("(#{line_item.unit_price_price_and_unit})")
+- if line_item.unit_price
+  = raw("(#{format_unit_price(line_item.unit_price)})")
+

app/views/spree/orders/_bought.html.haml

@@ -24,7 +24,7 @@
         = line_item.single_display_amount_with_adjustments.to_html
         %br
         %span.unit-price
-          = line_item.unit_price_price_and_unit
+          = format_unit_price(line_item.unit_price)
       %td.text-center.cart-item-quantity
         = line_item.quantity
       %td.cart-item-total.text-right

app/views/spree/orders/_line_item.html.haml

@@ -22,7 +22,7 @@
       = line_item.single_display_amount_with_adjustments.to_html
       %br
       %span.unit-price
-        = line_item.unit_price_price_and_unit
+        = format_unit_price(line_item.unit_price)
     %td.text-center.cart-item-quantity
       - finalized_quantity = @order.completed? ? line_item.quantity : 0
       = item_form.number_field :quantity,