Per review comment, Use named value on voucher submit button to distinguish between submission types

The voucher apply button is inside form that has another a submit button,
it leads to a weird situation where either one will submit the whole
payments page form. Adding a named parameter on the voucher apply button
means we can distinguish between the two by checking for the presence
of params[:apply_voucher].

app/controllers/split_checkout_controller.rb

@@ -30,11 +30,7 @@ class SplitCheckoutController < ::BaseController
   end
 
   def update
-    if add_voucher
-      return render_voucher_section_or_redirect
-    elsif @order.errors.present?
-      return render_error
-    end
+    return process_voucher if params[:apply_voucher].present?
 
     if confirm_order || update_order
       return if performed?
@@ -206,8 +202,19 @@ class SplitCheckoutController < ::BaseController
     selected_shipping_method.first.require_ship_address == false
   end
 
+  def process_voucher
+    if add_voucher
+      render_voucher_section_or_redirect
+    elsif @order.errors.present?
+      render_error
+    end
+  end
+
   def add_voucher
-    return unless payment_step? && params.dig(:order, :voucher_code).present?
+    if params.dig(:order, :voucher_code).blank?
+      @order.errors.add(:voucher, I18n.t('split_checkout.errors.voucher_not_found'))
+      return false
+    end
 
     # Fetch Voucher
     voucher = Voucher.find_by(code: params[:order][:voucher_code], enterprise: @order.distributor)

app/views/split_checkout/_voucher_section.cable_ready.haml

@@ -16,4 +16,4 @@
                 = t("split_checkout.step2.voucher.warning_forfeit_remaining_amount")
         - else
           = text_field_tag "[order][voucher_code]", params.dig(:order, :voucher_code), data: { action: "input->toggle-button-disabled#inputIsChanged", }, placeholder: t("split_checkout.step2.voucher.placeholder") , class: "voucher"
-          = submit_tag t("split_checkout.step2.voucher.apply"), disabled: true, class: "button cancel voucher", "data-disable-with": false, data: { "toggle-button-disabled-target": "button" }
+          = submit_tag t("split_checkout.step2.voucher.apply"), name: "apply_voucher", disabled: true, class: "button cancel voucher", "data-disable-with": false, data: { "toggle-button-disabled-target": "button" }

spec/controllers/split_checkout_controller_spec.rb

@@ -242,6 +242,7 @@ describe SplitCheckoutController, type: :controller do
         describe "adding a voucher" do
           let(:checkout_params) do
             {
+              apply_voucher: "true",
               order: {
                 voucher_code: voucher.code
               }
@@ -261,6 +262,7 @@ describe SplitCheckoutController, type: :controller do
           context "when voucher doesn't exist" do
             let(:checkout_params) do
               {
+                apply_voucher: "true",
                 order: {
                   voucher_code: "non_voucher"
                 }