Add a ``TransactionCase`` test suite covering the module behaviour:
- ``_define_user_id`` decision rules (no team, no current user, user not
in team, user in team, team without leader).
- ``create`` auto-assignment (with/without partner, with/without default
team, explicit team not overridden, partner without linked user, batch
``vals_list`` creation).
- ``_compute_user_id`` recomputation on team change.
- Full portal creation flow (team + project + user) and presence of the
``default_helpdesk_ticket_team_id`` field on ``res.users``.
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