Add Bugsnag notification if we reach tax rate refund code

The original Spree code allow for a tax adjustment to be considered a
refund in a specific scenario:
- instance is using inclusive tax
- instance that applies different tax rate in different tax zones

This scenario should not happen with how our instances are configured
More info: https://github.com/openfoodfoundation/openfoodnetwork/pull/6565#discussion_r566535431

app/models/spree/tax_rate.rb

@@ -105,6 +105,15 @@ module Spree
         if default_zone_or_zone_match?(item.order)
           calculator.compute(item)
         else
+          # Tax refund should not be possible with the way our production server are configured
+          Bugsnag.notify(
+            "Notice: Tax refund should not be possible, please check the default zone and " \
+            "the tax rate zone configuration"
+          ) do |payload|
+            payload.add_metadata :order_tax_zone, order.tax_zone
+            payload.add_metadata :tax_rate_zone, zone
+            payload.add_metadata :default_zone, Zone.default_tax
+          end
           # In this case, it's a refund.
           calculator.compute(item) * - 1
         end

spec/models/spree/tax_rate_spec.rb

@@ -386,6 +386,16 @@ module Spree
                 Spree::TaxRate.adjust(order, order.line_items)
                 expect(line_item.adjustments.credit.count).to eq 2
               end
+
+              it "notifies bugsnag" do
+                # there are two tax rate
+                expect(Bugsnag).to receive(:notify).with(
+                  "Notice: Tax refund should not be possible, please check the default zone and " \
+                  "the tax rate zone configuration"
+                ).twice
+
+                Spree::TaxRate.adjust(order, order.line_items)
+              end
             end
           end