Fix hierarchy widget pagination by prevent apollo cache mismatch

What does this MR do and why?

Fixes a bug where missing hasParent on Hierarchy widget fragment caused Apollo cache shape mismatch, leading to pagination not working on the widget.

References

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The hasParent field present in work_items/graphql/work_item_widgets.fragment.graphql#L52 but not being present in work_items/graphql/work_item_hierarchy.fragment.graphql#L24 was found when a diff was ran between contents of existingWidget & incomingWidget within graphql_shared/issuable_client.js#L465.

MR acceptance checklist

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Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Screen_Recording_2025-01-23_at_6.51.39_PM	Screen_Recording_2025-01-23_at_6.50.21_PM

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit an epic on your GDK with more than 5 child issues or epics present.
2. Go to work_items/constants.js#L253, and change value of DEFAULT_PAGE_SIZE_CHILD_ITEMS from 50 to 3. This will cause only 3 children to show on page load, and will show Load more button at the bottom of the hierarchy.
3. Click on Load more, it should load next batch of 3 children.
   • With the bug present, it loads children but hides it immediately and shows Load more again, and when you click on it once more, it'll show duplicates in the tree with console errors.

Related to #495532 (closed), #514562 (closed)