feat: add linked items to work item tool

What does this merge request do and why?

We need to be able to reason about the linked work items (linked to, blocked by, is blocking) when interacting with work item in Duo Planner.

How to set up and validate locally

How to set up and validate locally

required: DAP platform working locally

• in AI gateway, switch to branch from MR !3715
• in monolith, switch to the branch mk-add-ai-scope-to-widget from MR: gitlab-org/gitlab!210886 (merged)
• ask in agentic chat question like what are the linked items of http://gdk.test:3000/gitlab-duo/test/-/work_items/122 - work item needs to have linked items
• observe answer - it should list proper work items

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.