[FIX] contract: Fix invoice creation and salesperson assignment

In v13, the `user_id` field is a related field to `invoice_user_id`, that defaults to the environment user (`self.env.user`).
Therefore, if we try to create an invoice just by passing `user_id`, it would be overwritten by the default computation of `invoice_user_id`.
This fixes it by passing the correct field and data.

TT31715

contract/models/contract.py

@@ -411,7 +411,7 @@ class ContractContract(models.Model):
                 "invoice_date": date_invoice,
                 "journal_id": journal.id,
                 "invoice_origin": self.name,
-                "user_id": self.user_id.id,
+                "invoice_user_id": self.user_id.id,
             }
         )
         return invoice_vals, move_form