Prerequisites

    - Configure a mail server (incoming)
    - Enable leads in CRM app
    - Create a sales team with a mail alias (ex: testcrm@odoo.com)
      and check "Pipeline" and "Leads"
    - Set a partner email with one of yours (ex: bob@odoo.com) /!\
    - Send a mail from bob to testcrm
    - In the incoming mail server click on "Fetch"

Reproduce the issue:

    - Convert the generated lead to an opportunity
    - Choose "Existing customer" and set another one than the actual
    - The opportunity is created but the customer is the old one

Cause:

    The given partner_id is overriden by the old one in the
    `handle_partner_assignation` method.

This commit gives the priority to the new partner_id.

OPW-2089404

closes odoo/odoo#40031

X-original-commit: e1833936
Signed-off-by: Jason Van Malder <jasonvanmalder@users.noreply.github.com>