[FIX] hr_contract: Fix multi-company issue on api.constrains

Purpose
=======

Avoid mismatch between contracts from different companies and clarify
error message.

closes odoo/odoo#75553

closes odoo/odoo#120650

Related: odoo/enterprise#19647
Related: odoo/upgrade#2777
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/hr_contract/models/hr_contract.py

@@ -111,6 +111,7 @@ class Contract(models.Model):
             domain = [
                 ('id', '!=', contract.id),
                 ('employee_id', '=', contract.employee_id.id),
+                ('company_id', '=', contract.company_id.id),
                 '|',
                     ('state', 'in', ['open', 'close']),
                     '&',