[FIX] base_vat: keep leading zeros on San Marino tax id

### Steps to reproduce
* install the *VAT Number Validation `(base_vat)`* and Contacts module.
* Create a new contact from San Marino and set their Tax Id to SM05426
* Save the contact

You should see that the leading zeros have been removed on the Tax Id
(here, SM05426 became SM5426)

opw-3007722

closes odoo/odoo#103545

Signed-off-by: Josse Colpaert <jco@odoo.com>

addons/base_vat/models/res_partner.py

@@ -543,6 +543,10 @@ class ResPartner(models.Model):
         stdnum_vat_format = getattr(stdnum.util.get_cc_module('ch', 'vat'), 'format', None)
         return stdnum_vat_format('CH' + vat)[2:] if stdnum_vat_format else vat
 
+    def format_vat_sm(self, vat):
+        stdnum_vat_format = stdnum.util.get_cc_module('sm', 'vat').compact
+        return stdnum_vat_format('SM' + vat)[2:]
+
     def _fix_vat_number(self, vat, country_id):
         code = self.env['res.country'].browse(country_id).code if country_id else False
         vat_country, vat_number = self._split_vat(vat)