[REF] helpdesk_user_default_ticket_team: extract user assignment into `_define_user_id`

Extract the team-leader assignment logic into a dedicated
``_define_user_id`` helper, shared between ``_compute_user_id`` and
``create``:

- ``_compute_user_id`` now delegates to ``_define_user_id`` instead of
  inlining the membership/leader checks.
- ``create`` also assigns ``user_id`` from the default team via the same
  helper, so a portal-created ticket gets the team leader assigned.

This centralises the decision (keep current user / fall back to team
leader) in a single, reusable place.

helpdesk_user_default_ticket_team/models/helpdesk_ticket.py

@@ -7,10 +7,10 @@ class HelpdeskTicket(models.Model):
     @api.depends("team_id")
     def _compute_user_id(self):
         for ticket in self:
-            if ticket.team_id:
-                if ticket.user_id not in ticket.team_id.user_ids:
-                    if ticket.team_id.user_id:
-                        ticket.user_id = ticket.team_id.user_id
+            ticket.user_id = self._define_user_id(
+                ticket.team_id,
+                ticket.user_id,
+            )
 
     @api.model_create_multi
     def create(self, vals_list):
@@ -35,4 +35,20 @@ class HelpdeskTicket(models.Model):
                 if team.default_project_id:
                     vals["project_id"] = team.default_project_id.id
 
+                # Set the user_id to which the ticket is assigned
+                user_id = self._define_user_id(team, None)
+                if user_id:
+                    vals["user_id"] = user_id.id
         return super().create(vals_list)
+
+    def _define_user_id(self, team_id=None, ticket_user_id=None):
+        if not team_id:
+            return ticket_user_id
+
+        if not ticket_user_id:
+            return team_id.user_id
+
+        if ticket_user_id not in team_id.user_ids:
+            return team_id.user_id
+
+        return ticket_user_id