Add foreign keys to shipping method related models

The `Spree::ShippingMethodCategory` is a join model and needs foreign
keys on both ends. The shipping method id on products was mandatory
before, so it's safe to remove the `optional: true` on the `belongs_to`
declaration and add a foreign key constraint here as well.

Author: mamhoff

core/app/models/spree/product.rb

@@ -31,7 +31,7 @@ class Product < Spree::Base
     has_many :taxons, through: :classifications, before_remove: :remove_taxon
 
     belongs_to :tax_category, class_name: 'Spree::TaxCategory', optional: true
-    belongs_to :shipping_category, class_name: 'Spree::ShippingCategory', inverse_of: :products, optional: true
+    belongs_to :shipping_category, class_name: 'Spree::ShippingCategory', inverse_of: :products
     belongs_to :primary_taxon, class_name: 'Spree::Taxon', optional: true
 
     has_one :master,
@@ -122,7 +122,6 @@ def find_or_build_master
     validates :meta_title, length: { maximum: 255 }
     validates :name, presence: true
     validates :price, presence: true, if: proc { Spree::Config[:require_master_price] }
-    validates :shipping_category_id, presence: true
     validates :slug, presence: true, uniqueness: { allow_blank: true, case_sensitive: true }
 
     attr_accessor :option_values_hash

core/app/models/spree/shipping_method_category.rb

@@ -2,7 +2,7 @@
 
 module Spree
   class ShippingMethodCategory < Spree::Base
-    belongs_to :shipping_method, class_name: 'Spree::ShippingMethod', optional: true
-    belongs_to :shipping_category, class_name: 'Spree::ShippingCategory', inverse_of: :shipping_method_categories, optional: true
+    belongs_to :shipping_method, class_name: 'Spree::ShippingMethod'
+    belongs_to :shipping_category, class_name: 'Spree::ShippingCategory', inverse_of: :shipping_method_categories
   end
 end

core/db/migrate/20250605105424_add_shipping_category_foreign_keys.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class AddShippingCategoryForeignKeys < ActiveRecord::Migration[7.0]
+  def change
+    add_foreign_key :spree_products, :spree_shipping_categories, column: :shipping_category_id, null: false
+    add_foreign_key :spree_shipping_method_categories, :spree_shipping_methods, column: :shipping_method_id, null: false
+    add_foreign_key :spree_shipping_method_categories, :spree_shipping_categories, column: :shipping_category_id, null: false
+  end
+end