[FIX] hr_holidays: allow message _subscribe to work in batch

In case more that one leave record in passed to message _subscribe
in will throw "Expected singleton" ValueError.

This commit fixed the issue.

Part-of: odoo/odoo#142327

addons/hr_holidays/models/hr_leave.py

@@ -1596,7 +1596,7 @@ Attempting to double-book your time off won't magically make your vacation 2x be
 
     def message_subscribe(self, partner_ids=None, subtype_ids=None):
         # due to record rule can not allow to add follower and mention on validated leave so subscribe through sudo
-        if self.state in ['validate', 'validate1']:
+        if any(holiday.state in ['validate', 'validate1'] for holiday in self):
             self.check_access_rights('read')
             self.check_access_rule('read')
             return super(HolidaysRequest, self.sudo()).message_subscribe(partner_ids=partner_ids, subtype_ids=subtype_ids)