Add GraphQL endpoints for workflow-pipeline links

What does this MR do and why?

Adds GraphQL support for linking GitLab Duo Agent Platform workflow sessions to pipelines (both directions):

1. Allows fetching all pipelines for a session (pipelineLinks on DuoWorkflow)
2. Allows fetching all sessions from a pipeline (duoWorkflowLinks on Pipeline)

This mirrors the pattern established for merge request links, notes, and work items.

References

Relates to #603665 (closed)

How to set up and validate locally

1. Start a fix_pipeline flow against a failing pipeline. Note the workflow ID (from the session info) and the pipeline ID (from the pipeline URL)
2. Get the GID for the pipeline:

pipeline_id = <MY_PIPELINE_ID_FROM_PIPELINE_URL>
Ci::Pipeline.find(pipeline_id).to_global_id

3. Confirm the link from the pipeline to the session is available via GraphQL, and the linked workflow matches the noted ID above:

query PipelineWorkflowLinks($pipelineId: CiPipelineID!) {
  project(fullPath: "gitlab-duo/test") {
    pipeline(id: $pipelineId) {
      duoWorkflowLinks {
        nodes {
          linkType
          workflow { id }
        }
      }
    }
  }
}

example output:

{
  "data": {
    "project": {
      "pipeline": {
        "duoWorkflowLinks": {
          "nodes": [
            {
              "linkType": "SOURCE",
              "workflow": {
                "id": "gid://gitlab/Ai::DuoWorkflows::Workflow/4772"
              }
            }
          ]
        }
      }
    }
  },
  "correlationId": "01M0D7B85TYYDMC1HDZZ0YP10W"
}

4. Confirm the pipelines are available from the session:

query WorkflowPipelineLinks {
  duoWorkflowWorkflows(workflowId: "gid://gitlab/Ai::DuoWorkflows::Workflow/4772") {
    nodes {
      id
      pipelineLinks {
        nodes {
          linkType
          pipeline { id iid }
        }
      }
    }
  }
}

example output:

{
  "data": {
    "duoWorkflowWorkflows": {
      "nodes": [
        {
          "id": "gid://gitlab/Ai::DuoWorkflows::Workflow/4772",
          "pipelineLinks": {
            "nodes": [
              {
                "linkType": "SOURCE",
                "pipeline": {
                  "id": "gid://gitlab/Ci::Pipeline/6582",
                  "iid": "362"
                }
              }
            ]
          }
        }
      ]
    }
  },
  "correlationId": "01M0D7H11A777SX9XG8B2DD0YR"
}

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