[FIX] account_banking_ach_base: use @api.model_create_multi for batch-aware create

- account_banking_ach_base/models/account_banking_mandate.py

Co-Authored-By: Don Kendall <kendall@donkendall.com>

Author: natuan9

account_banking_ach_base/models/account_banking_mandate.py

@@ -56,8 +56,9 @@ def set_payment_modes_on_partner(self):
         if payment_modes:
             self.partner_id.write(payment_modes)
 
-    @api.model
-    def create(self, vals):
-        mandate = super(AccountBankingMandate, self).create(vals)
-        mandate.set_payment_modes_on_partner()
-        return mandate
+    @api.model_create_multi
+    def create(self, vals_list):
+        mandates = super(AccountBankingMandate, self).create(vals_list)
+        for mandate in mandates:
+            mandate.set_payment_modes_on_partner()
+        return mandates