Add user column to audit report table and session details panel

What does this MR do and why?

Adds a User column to the audit report table and a User row to the session details panel, displaying the user who triggered the session.

The backend already exposes a triggeredBy field on DuoWorkflowSessionArtifact (https://gitlab.com/gitlab-org/gitlab/-/work_items/606639). This MR wires up the frontend to display that data.

Changes:

• Add triggeredBy { id name username avatarUrl webPath } to both group and project agent artifacts GraphQL queries
• Add a User column (avatar + name link) to the audit report table (agent_artifacts_table.vue)
• Add a User row (avatar + name link) to the session details panel (session_details_body.vue)
• Update mock data and specs to cover both the populated and null cases (shows — when triggeredBy is null)
• Update locale/gitlab.pot with the new AgentArtifacts|User translation string

References

• https://gitlab.com/gitlab-org/gitlab/-/work_items/607541+
• https://gitlab.com/gitlab-org/gitlab/-/work_items/606639+

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to a group or project's AI audit report
2. Verify the User column appears in the table with avatar and name link
3. Click a session row to open the details panel
4. Verify the User row appears in the session details with avatar and name link
5. Verify null user cases show a — fallback

MR acceptance checklist

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