Populate the list of parents from ancestors to filter work items

What does this MR do and why?

This MR populates parent work items on the work items list filter. It is part of series of MR for the task #556200 (closed)

• Add feature flag and show parent token based on the flag
• Show list of epics to filter the work items based on if it has a parent -> This MR
• Filter the work items based on the selected parent in the filter

References

• Related to #556200 (closed)
• Parent issue #536876 (closed)
• Feature epic &16340 (closed)

Screenshots or screen recordings

The list simply has epics, issues and objectives on project level while epics on the group level.

	Recording
Group epics list	Screen Recording 2025-08-21 at 1.13.04 AM.mov
Project issue list	Screen Recording 2025-08-21 at 1.12.09 AM.mov

How to set up and validate locally

Prerequisites:

1. For work items list, enable FF work_item_planning_view
2. For project issue list with work item view, enable FF work_items_project_issues_list
3. To enable parent filter, use FF work_items_list_parent_filter

Steps:

1. Go to Group > Work items or Project > Work items or Project > Issues based on the feature flag
2. Verify if the Parent filter is available based if work_items_list_parent_filter is turned on
3. Select the Parent token and verify if the list of parent are from current namespace and/or ancestor group

This also works with work items list.

MR acceptance checklist

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Related to #556200 (closed)