refactor(products): keep relation names on GraphQL errors

## Context

GraphQL product mutations remapped model association errors to the id
input fields (billable_metric_id, product_category_id) through a
SurfaceErrorFields concern. The FE owns its error-display mapping, so
the neutral relation name is enough on GraphQL — while REST keeps
translating to the _code params its callers actually send, where the
contract is public.

## Description

Drop the SurfaceErrorFields concern: GraphQL validation errors now
carry the relation name emitted by the model. The REST boundary keeps
its billable_metric_code and product_category_code mapping.

Author: rsempe

app/graphql/mutations/products/create.rb

@@ -5,7 +5,6 @@ module Products
     class Create < BaseMutation
       include AuthenticableApiUser
       include RequiredOrganization
-      include SurfaceErrorFields
 
       REQUIRED_PERMISSION = "products:create"
 
@@ -18,7 +17,7 @@ class Create < BaseMutation
       def resolve(**args)
         result = ::Products::CreateService.call(organization: current_organization, params: args)
 
-        result.success? ? result.product : render_item_error(result)
+        result.success? ? result.product : result_error(result)
       end
     end
   end

app/graphql/mutations/products/surface_error_fields.rb

@@ -5,7 +5,6 @@ module Products
     class Update < BaseMutation
       include AuthenticableApiUser
       include RequiredOrganization
-      include SurfaceErrorFields
 
       REQUIRED_PERMISSION = "products:update"
 
@@ -19,7 +18,7 @@ def resolve(**args)
         product = current_organization.products.find_by(id: args[:id])
         result = ::Products::UpdateService.call(product:, params: args.except(:id))
 
-        result.success? ? result.product : render_item_error(result)
+        result.success? ? result.product : result_error(result)
       end
     end
   end

app/graphql/mutations/products/update.rb

@@ -71,8 +71,8 @@
   context "when a usage item has no billable metric" do
     let(:input) { {name: "Orphan", code: "orphan", productType: "usage"} }
 
-    it "returns a validation error on billable_metric_id" do
-      expect(execution["errors"].first["extensions"]["details"]).to eq("billableMetricId" => ["value_is_mandatory"])
+    it "returns a validation error on the relation" do
+      expect(execution["errors"].first["extensions"]["details"]).to eq("billableMetric" => ["value_is_mandatory"])
     end
   end
 end