Chore: Consolidate WorkItemQueryBuilder for Project and Group Work Items

What does this MR do and why?

This change introduces a new feature flag called search_scope_work_item that allows switching between two different search methods for Epics.

When the feature flag is enabled, the system uses a WorkItemQueryBuilder, that can now handle both project-level and group-level searches. When disabled, it falls back to the WorkItemGroupQueryBuilder for group searches only. This will enable us to remove the WorkItemGroupQueryBuilder and also enable us to implement the new UI for work_items

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

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 &16459