[IMP] mail: improve get_mention_suggestion performance

"Order by" makes the query really slow.

On a database with 1000000 partner and 10000 user, this query goes from 5.5s to
600ms. Once cached by Pgsql it goes down below 100ms.

task-2982401

closes odoo/odoo#118198

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>
Co-authored-by: Sébastien Theys <seb@odoo.com>

addons/mail/models/res_partner.py

@@ -335,7 +335,11 @@ class Partner(models.Model):
             remaining_limit = limit - len(partners)
             if remaining_limit <= 0:
                 break
-            partners |= self.search(expression.AND([[('id', 'not in', partners.ids)], domain]), limit=remaining_limit)
+            # We are using _search to avoid the default order that is
+            # automatically added by the search method. "Order by" makes the query
+            # really slow.
+            query = self._search(expression.AND([[('id', 'not in', partners.ids)], domain]), limit=remaining_limit)
+            partners |= self.browse(query)
         partners_format = partners.mail_partner_format()
         if channel_id:
             member_by_partner = {member.partner_id: member for member in self.env['mail.channel.member'].search([('channel_id', '=', channel_id), ('partner_id', 'in', partners.ids)])}