Refactored WorkItemTokenInput to rely on reactive variables

What does this MR do and why?

This MR refactors WorkItemTokenInput component:

• removed unused this.query property in favor of searchTerm
• uses correct method for focusing token selector input
• refactors explicit refetch and simple query to use smart queries with reactive variables
• adds loading state to work items by reference search

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.

How to set up and validate locally

1. Enable the OKRs feature as mentioned in the OKRs docs (e.g. ::Feature.enabled?(:okrs_mvc, Project.find(<PROJECT_ID>)) and the namespace must have Ultimate plan).
2. Enable the feature flag Feature.enable(:linked_work_items)
3. Make sure that adding linked items to work items work with both parent workspace and adding a child by link.