Add markdown to agent sessions rake task

What does this MR do and why?

Add markdown to agent sessions rake task

To have better testing for agent sessions, we want to have markdown content to make sure it properly renders.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run bundle exec rake "gitlab:duo_workflow:populate[5,5,gitlab-duo/test]
2. Refresh
3. The last 5 agent sessions you see should have markdown content!

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.