[FIX] pos_mercury: take into account rescue sessions

If there are open rescue sessions there will be multiple opened
pos.session records with the same user.

A better solution would be to make the POS client send back the
pos.session id it belongs to and use that instead, but this would be
unsuitable for a stable release. It will instead be done in master.

opw-743318

addons/pos_mercury/models/pos_mercury_transaction.py

@@ -15,7 +15,7 @@ class MercuryTransaction(models.Model):
     _name = 'pos_mercury.mercury_transaction'
 
     def _get_pos_session(self):
-        pos_session = self.env['pos.session'].search([('state', '=', 'opened'), ('user_id', '=', self.env.uid)])
+        pos_session = self.env['pos.session'].search([('state', '=', 'opened'), ('user_id', '=', self.env.uid)], limit=1)
         if not pos_session:
             raise UserError("No POS session")
 
@@ -32,6 +32,7 @@ class MercuryTransaction(models.Model):
             raise UserError("No Mercury configuration associated with the journal.")
 
     def _setup_request(self, data):
+        # todo: in master make the client include the pos.session id and use that
         pos_session = self._get_pos_session()
 
         config = pos_session.config_id