[IMP] account_tax: add country_code for use in domains

closes odoo/odoo#86236

Related: odoo/enterprise#25187
Signed-off-by: Laurent Smet <las@odoo.com>

addons/account/models/account_tax.py

@@ -38,6 +38,7 @@ class AccountTaxGroup(models.Model):
         string='Advance Tax payment account',
         help="Downpayments posted on this account will be considered by the Tax Closing Entry.")
     country_id = fields.Many2one(string="Country", comodel_name='res.country', help="The country for which this tax group is applicable.")
+    country_code = fields.Char(related="country_id.code")
     preceding_subtotal = fields.Char(
         string="Preceding Subtotal",
         help="If set, this value will be used on documents as the label of a subtotal excluding this tax group before displaying it. " \

addons/account/views/account_tax_views.xml

@@ -215,6 +215,7 @@
                     <field name="sequence" widget="handle"/>
                     <field name="name"/>
                     <field name="country_id"/>
+                    <field name="country_code" invisible="True"/>
 
                     <!--
                         'force_account_company' context key is used so that the RedirectWarning