[WebAgenticChat] Send current AI context when workflow is started

Overview

Web Agentic Chat should be aware of the current resource that is being viewed (merge request, issue, etc...).

Classic chat uses resource_id for this purpose, which is a global id of a resource (like gid://gitlab/MergeRequest/123)

We cannot pass a global id as a context, because some of the resources (like merge requests and issues) can be fetched via API only by specifying project_id + iid

Proposal

Send current AI context as the additionalContext field to this request