Paginate the Duo session chat log and cap duoMessages

What does this MR do and why?

Paginate the Duo session chat log and cap duoMessages

Reading a session's chat log through duoMessages returned every message with no bound, and the session page polled it every 10 seconds.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/635073

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable FF duo_workflow_cap_duo_messages
2. Run a workflow that takes multiple steps (e.g. ask Developer @duo-developer-gitlab-duo - Write a python script that tells me the time, but it's always wrong.)
3. Check you can get the messages, newest first, with pagination, within messages:

  query testCheckPointPagination{
    duoWorkflowWorkflows(workflowId: "gid://gitlab/Ai::DuoWorkflows::Workflow/4960") {
      nodes {
        id
        latestCheckpoint {
          threadTs
          duoMessages { content role threadTs parentTs alternativeCount }
          messages(last: 2) {
            nodes { content messageType role threadTs }
            pageInfo { hasPreviousPage hasNextPage startCursor endCursor }
          }
        }
      }
    }
  }

4. To test the cap, set DUO_MESSAGES_CAP in workflow_event_type.rb to 2, and rerun the above query. The duoMessages should be limited to 2 with the FF on, and not limited with it off.

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