Add inverse_of to state/address, add FK constraint

For the database, a state is optional on an address. What this commit
ensures is that any state ID entered into the `state_id` column on
`spree_addresses` is actually present in the `spree_states` table.

Author: mamhoff

core/app/models/spree/state.rb

@@ -3,7 +3,7 @@
 module Spree
   class State < Spree::Base
     belongs_to :country, class_name: 'Spree::Country'
-    has_many :addresses, dependent: :nullify
+    has_many :addresses, dependent: :nullify, inverse_of: :state
 
     validates :name, presence: true
 

core/db/migrate/20250709084513_add_state_foreign_keys.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class AddStateForeignKeys < ActiveRecord::Migration[7.0]
+  FOREIGN_KEY_VIOLATION_ERRORS = %w[PG::ForeignKeyViolation Mysql2::Error SQLite3::ConstraintException]
+
+  def up
+    # Uncomment the following code to remove orphaned records if this migration fails
+    #
+    # say_with_time "Resetting state IDs on addresses where the state record is no longer present" do
+    #   Spree::Address.where.not(state_id: nil).left_joins(:state).where(spree_states: { id: nil }).update_all(state_id: nil)
+    # end
+
+    add_foreign_key :spree_addresses, :spree_states, column: :state_id, null: true, on_delete: :restrict
+  rescue ActiveRecord::StatementInvalid => e
+    if e.cause.class.name.in?(FOREIGN_KEY_VIOLATION_ERRORS)
+      say <<~MSG
+        ⚠️ Foreign key constraint failed when adding :spree_addresses => :spree_states.
+        To fix this:
+          1. Uncomment the code that removes orphaned records.
+          2. Rerun the migration.
+        Offending error: #{e.cause.class} - #{e.cause.message}
+      MSG
+    end
+    raise
+  end
+
+  def down
+    remove_foreign_key :spree_addresses, :spree_states, column: :state_id, null: true, on_delete: :restrict
+  end
+end