Extract works item subscribed field to a separate query

What does this MR do and why?

We are observing many workItemByIidQuery queries exceeding the target duration of 5s, which impacts the error budget for groupproduct planning and groupproject management. We can assume that the subscribed field in the Notifications widget might be responsible for some of these failures because they can be linked to work items with a large number of notes (which make the field calculation slow).

In an attempt to remove this expensive calculation from workItemByIidQuery and workItemByIdQuery, these changes move the widget to a separate query getWorkItemNotificationsById.

References

Related to #553194 (closed)

Screenshots or screen recordings

Screen_Recording_2025-07-24_at_17.13.50

How to set up and validate locally

1. Visit a work item and verify that the query getWorkItemNotificationsbyId is executed when the page loads
2. Toggle the subscription and verify that the new value is displayed after the mutation is completed

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.