Show human author in Agent artifacts audit event summary and filter from details

What does this MR do and why?

Adds human author rows to the audit event Summary section and filters human_author_* keys from the Details section for Agent artifacts audit events.

Non-composite events (no humanAuthor) show neither row, rather than an em dash.

References

• Parent issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/607616
• Task: https://gitlab.com/gitlab-org/gitlab/-/work_items/614158
• Previous iteration (surfaced humanAuthor in the list): https://gitlab.com/gitlab-org/gitlab/-/issues/611656

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the agent_artifacts_page beta flag.
2. Navigate to an Agent artifacts session that has composite audit events (service account acting on behalf of a human).
3. Click an audit event to open the details panel.
4. Verify the Summary section shows "Human author ID" and "Human author name" rows.
5. Verify the Details section does not show Human author id, Human author name, or Human author username rows.
6. Click a non-composite event and verify no human author rows appear in Summary.

MR acceptance checklist

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