Draft: Filters Child items by state using GraphQL

What does this MR do and why?

Child items were being filtered based on its state using frontend logic. This was creating issues like mentioned in pagination. This MR

• simplifies by using filters by state in GraphQL queries itself. This not only resolves our problem but if the toggle is kept off, it improves performance by fetching only the open item data from the server rather than filtering on the client side.
• Show n closed items is changed to Show closed items
• The changes affect both the work item tree view and the relationships view.
• As the button do not have other logic, the shared component WorkItemToggleClosedItems is also removed.

References

• Related to #516121

Screenshots or screen recordings

Show closed items toggle	Before	After
Enabled	Screen Recording 2025-05-14 at 9.49.06 PM.mov	Screen Recording 2025-05-14 at 9.24.56 PM.mov
Disabled	Screen Recording 2025-05-14 at 9.51.30 PM.mov	Screen Recording 2025-05-14 at 9.55.20 PM.mov

How to set up and validate locally

1. Create an epic with more than 5 child issues or epics present.
2. To paginate with lesser items, use the query param http://gdk.test:3000/groups/gitlab-org/-/epics/165?children_count=4
3. Click on Load more, it should load next batch of 4 children.
4. Verify if Load more does not need to be pressed multiple times to show Show closed items

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.

Related to #516121