Portal users don't have write/create permissions on sale.order by default.
This causes errors when trying to create orders during checkout or draft save.
Changes:
- Add _get_salesperson_for_order() helper to retrieve partner's salesperson
- Use sudo() for all sale.order create() operations
- Automatically assign user_id (salesperson) when creating orders
- Use sudo() for order updates and line modifications
- Add fallback to commercial_partner_id.user_id for salesperson
This ensures orders are created with proper permissions while maintaining
traceability through the assigned salesperson.
Test coverage:
- Add test_portal_sale_order_creation.py with 3 tests
- Test portal user creates sale.order
- Test salesperson fallback logic
- Test portal user updates order lines
