Hide stock level of on-demand inventory items

We changed to tracking stock of on-demand items to be able to place
backorders. This is mostly hidden in the app but was still visible on
the inventory page. Now we are hiding that here, too.

app/controllers/admin/variant_overrides_controller.rb

@@ -71,6 +71,12 @@ module Admin
 
     def load_collection
       collection_hash = Hash[variant_overrides_params.each_with_index.map { |vo, i| [i, vo] }]
+
+      # Reset count_on_hand when switching to producer settings:
+      collection_hash.each_value do |vo|
+        vo["count_on_hand"] = nil if vo.fetch("on_demand", :unchanged).nil?
+      end
+
       @vo_set = Sets::VariantOverrideSet.new(@variant_overrides,
                                              collection_attributes: collection_hash)
     end

app/serializers/api/admin/variant_override_serializer.rb

@@ -6,6 +6,12 @@ module Api
       attributes :id, :hub_id, :variant_id, :sku, :price, :count_on_hand, :on_demand,
                  :default_stock, :resettable, :tag_list, :tags, :import_date
 
+      def count_on_hand
+        return if object.on_demand
+
+        object.count_on_hand
+      end
+
       def tag_list
         object.tag_list.join(",")
       end

app/serializers/api/admin/variant_simple_serializer.rb

@@ -19,6 +19,7 @@ module Api
       end
 
       def on_hand
+        return if object.on_demand
         return 0 if object.on_hand.nil?
 
         object.on_hand

spec/system/admin/variant_overrides_spec.rb

@@ -255,7 +255,8 @@ RSpec.describe "
         context "with overrides" do
           let!(:vo) {
             create(
-              :variant_override, :on_demand,
+              :variant_override,
+              on_demand: true, count_on_hand: -5,
               variant:, hub:, price: 77.77, default_stock: 1000, resettable: true,
               tag_list: ["tag1", "tag2", "tag3"]
             )