Issue:
When updating partner fields such as `email`, `name` and `phone` on the partner records,
the ticket fields are recomputed. During the recomputation, the `description` field is loaded into memory and with
many helpdesk tickets, this will cause a memory error.

Purpose of this PR:
To explicity fetch `partner_id` before computing partner fields on the helpdesk ticket.

Example:
With a partner record with 25,000 helpdesk tickets each with a description of varying text and images,
we hit the memory limit when trying to update the partner phone. With these changes, peak memory usage reached 271.4 MB.

Notes:
Originally wanted to avoid changing the field definition however this would require adding `with_context(prefetch_fields=False)`
to most of the compute methods on helpdesk ticket model.`with_context(prefetch_fields=False)` to most of the compute methods on helpdesk ticket model.

Memgraph and Stats before changes
<img width="1912" height="862" alt="memgraph_b4_changes" src="https://github.com/user-attachments/assets/f5e95a42-83c8-4250-bdf3-440740a9fb33" />
<img width="640" height="352" alt="stats_b4_changes" src="https://github.com/user-attachments/assets/a82ccd69-1359-4ba8-81a0-52aeace762db" />


opw-5069424

Forward-Port-Of: odoo/enterprise#96198