Add work_items_list_parent_filter FF for the Parent filter

What does this MR do and why?

This MR adds the feature flag and hide and show the Parent filter token on the project issue list, epics list and work items list based on feature flags. It is part of series of MR for the task #556200 (closed)

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

Screen Recording 2025-08-20 at 12.13.25 AM.mov

How to set up and validate locally

Prerequiresites:

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

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 #556200 (closed)