Fix refetching on task links after adding a new task

What does this MR do and why?

Currently, as described in !103564 (comment 1169371185), there is a work item refetch after we add an existing task to the work item. This happens because the mutation response contains just a partial result comparing to the query, so the cache gets invalidated => Apollo performs a refetch. This MR aligns query and mutation responses and prevents cache invalidation.

Screenshots or screen recordings

No visual changes

How to set up and validate locally

1. Open an issue page and open Network tab in browser devtools.
2. Add a new Task to the tasks widget, when adding, select Adding an existing task.
3. Make sure that after workItemUpdate mutation there is no workItem query call

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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