[18.0][FIX] helpdesk_mgmt_project: explicitly assign project to task

[github-yotech]

Problem Description
When creating a new task from the task_id field on a helpdesk.ticket form, the task is created as a "Private Task" (assigned to the user's private project) instead of adopting the project already selected on the ticket.

Expected Behavior
Any new task created and linked to a ticket should explicitly inherit the project_id from the Helpdesk Ticket.

Root Cause
The previous implementation relied solely on the UI passing default_project_id via context. However, duplicate project_id field definitions in the XML (one hidden, one visible) caused the Odoo 18 Web Client to fail during context evaluation, resulting in the task being created without a project. Furthermore, the backend did not explicitly enforce the project synchronization upon task linkage.

Solution
Following OCA guidelines prioritizing Explicit over Implicit behavior:

1. Backend Fix: Overrode create and write on helpdesk.ticket. If a task is linked that does not have a project_id, it is now explicitly assigned the ticket's project_id.
2. UI Cleanup: Removed the redundant invisible="1" duplicate field definitions in helpdesk_ticket_view_form to prevent Web Client evaluation issues.
3. Tests: Updated the unit test to deliberately create a task without context and verify that the backend correctly and explicitly assigns the project upon linking it to the ticket.