Create links between workflows and work_items

What does this MR do and why?

Stacked MR, that uses the tables created in !239375 (merged)

This MR adds the model-level layer on top of the join tables, for workflow -> work_items

• Added the Ai::DuoWorkflows::WorkflowWorkItem model on the workflow to work-items join table, with a link_type enum (Two link types added here: source, and created).
• Extracted shared join-model behavior into the Ai::DuoWorkflows::WorkflowLinkable concern (associations, validations, and a links_workflow_to macro).
• Added the generic Ai::DuoWorkflows::LinkArtifactService as the single entry point for creating workflow-artifact links, dispatching to the right join model by artifact type.
• Leverages the header added in gitlab-org#22201 (closed) to track any work items created by a flow.

References

Starts the linking for #600655 (closed)

Uses the tables created in #599435 (closed)

How to set up and validate locally

1. Go to an issue in your GDK
2. Add a comment tagging developer, telling it to create a new issue, e.g. @duo-developer-gitlab-duo Create an issue with this title: "Create a list of cars in cars.txt"
3. In your rails console, confirm the link was created from the source issue, and to the created issue:

[20] pry(main)> workflow = Ai::DuoWorkflows::Workflow.last

[21] pry(main)> workflow.linked_work_items
...
=> [#<WorkItem id:759 gitlab-duo/ai-assist-2#6>, #<WorkItem id:761 gitlab-duo/ai-assist-2#8>]

[22] pry(main)> workflow.work_item_links
...
=> [#<Ai::DuoWorkflows::WorkflowWorkItem:0x0000000102e09108
  id: 3,
  workflow_id: 4536,
  work_item_id: 759,
  project_id: 78,
  namespace_id: nil,
  created_at: "2026-06-19 22:04:48.766891000 +0000",
  updated_at: "2026-06-19 22:04:48.766891000 +0000",
  link_type: "source">,
 #<Ai::DuoWorkflows::WorkflowWorkItem:0x0000000102e08fc8
  id: 4,
  workflow_id: 4536,
  work_item_id: 761,
  project_id: 78,
  namespace_id: nil,
  created_at: "2026-06-19 22:05:58.106203000 +0000",
  updated_at: "2026-06-19 22:05:58.106203000 +0000",
  link_type: "created">]

[23] pry(main)> workflow.work_item_links.where(link_type: :source)
...
=> [#<Ai::DuoWorkflows::WorkflowWorkItem:0x0000000139ba9d90
  id: 3,
  workflow_id: 4536,
  work_item_id: 759,
  project_id: 78,
  namespace_id: nil,
  created_at: "2026-06-19 22:04:48.766891000 +0000",
  updated_at: "2026-06-19 22:04:48.766891000 +0000",
  link_type: "source">]

[24] pry(main)> workflow.work_item_links.where(link_type: :created)
...
=> [#<Ai::DuoWorkflows::WorkflowWorkItem:0x0000000139ba4890
  id: 4,
  workflow_id: 4536,
  work_item_id: 761,
  project_id: 78,
  namespace_id: nil,
  created_at: "2026-06-19 22:05:58.106203000 +0000",
  updated_at: "2026-06-19 22:05:58.106203000 +0000",
  link_type: "created">]

We can also check the inverse:

[4] pry(main)> project = workflow.project

[5] pry(main)> work_item   = project&.work_items&.find_by_iid(6)

[6] pry(main)> work_item.linked_duo_workflows
...
=> [#<Ai::DuoWorkflows::Workflow:0x0000000138376688
  id: 4536,
  user_id: 1,
  project_id: 78,

An additional test: Click "Implement work item" on a work_item and validate that it has a source link:

> workflow.work_item_links

MR acceptance checklist

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