Make work item child paths IID or full path

What does this MR do and why?

Previously child paths followed different logic based on level:

• Top-level children would have the parent namespace subtracted from their path, e.g. gitlab-org parent and gitlab-org/gitlab child resulted in /gitlab
• Children below top level showed full path always

This MR:

• Applies the same logic to all levels of hierarchy
• Shows either the full path, if in a different namespace, or just <#IID> if in the same namespace, mirroring the reference pattern used elsewhere

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

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

1. Enable work item feature flags: namespace_level_work_items work_item_epics_rollout
2. Create and open a work item epic
3. Add children in the same group and in different groups/projects

Closes #473218 (closed)