[IMP] project: search on child contacts too

When filtering tasks by partner, search also on the contacts of a company

Closes #27118

addons/project/views/project_views.xml

@@ -23,7 +23,7 @@
                <search string="Tasks">
                     <field name="name" string="Tasks"/>
                     <field name="tag_ids"/>
-                    <field name="partner_id"/>
+                    <field name="partner_id" operator="child_of"/>
                     <field name="project_id"/>
                     <field name="user_id"/>
                     <field name="stage_id"/>