Add configurable mergeable orders finder class

Introduce `Spree::Config.mergeable_orders_finder_class` to allow
stores to customize which orders are merged when a user logs in.
By default, all incomplete orders from the same store are merged,
but stores can implement custom logic (e.g., only merge orders
from the last 7 days) by providing a custom finder class.

Changes include:

- Add `Spree::MergeableOrdersFinder` to centralize order selection logic
  and support custom strategies
- Update `set_current_order` in `Spree::Core::ControllerHelpers::Order`
  to delegate order selection to the configured finder
- Add `class_name_attribute :mergeable_orders_finder_class` to
  `Spree::AppConfiguration` with default `'Spree::MergeableOrdersFinder'`

Issue #6366

Signed-off-by: Thukten Singye <[email protected]>

Author: ThuktenSingye

core/app/helpers/spree/core/controller_helpers/order.rb

@@ -42,10 +42,12 @@ def associate_user
         end
 
         def set_current_order
-          if spree_current_user && current_order
-            spree_current_user.orders.by_store(current_store).incomplete.where('id != ?', current_order.id).find_each do |order|
-              current_order.merge!(order, spree_current_user)
-            end
+          Spree::Config.mergeable_orders_finder_class.new(
+            user: spree_current_user,
+            store: current_store,
+            current_order: current_order
+          ).call.find_each do |order|
+            current_order.merge!(order, spree_current_user)
           end
         end
 

core/app/models/spree/mergeable_orders_finder.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Spree
+  class MergeableOrdersFinder
+    def initialize(user:, store:, current_order:)
+      @user = user
+      @store = store
+      @current_order = current_order
+    end
+
+    def call
+      @user.orders.by_store(@store).incomplete.where.not(id: @current_order.id)
+    end
+  end
+end

core/lib/spree/app_configuration.rb

@@ -409,6 +409,8 @@ def default_pricing_options
     #   as Spree::OrderMerger.
     class_name_attribute :order_merger_class, default: 'Spree::OrderMerger'
 
+    class_name_attribute :mergeable_orders_finder_class, default: 'Spree::MergeableOrdersFinder'
+
     # Allows providing your own class for adding default payments to a user's
     # order from their "wallet".
     #