Remove the sync_ai_governance_sessions feature flag

What does this MR do and why?

Removes the sync_ai_governance_sessions feature flag. SyncSessionArtifactWorker now always writes ai_governance_sessions from Duo workflows, on GitLab.com and self-managed.

The flag was enabled for everyone on GitLab.com on 2026-10-01 after production data checks showed every new workflow got a session with matching fields.

Known gap, tracked separately: session status is not updated when a workflow is failed by CleanStuckWorkflowsService (it calls workflow.drop directly). This MR does not change that.

Draft until the flag has run fully on for a few days. Merge is planned for the week of 2026-10-05.

References

• Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/630766
• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/630096
• Epic: https://gitlab.com/groups/gitlab-org/-/work_items/21540
• Tracking issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/630862
• Flag introduced in: !256733 (merged)

Screenshots or screen recordings

Not applicable, backend only.

How to set up and validate locally

1. Run a Duo flow in your GDK.
2. Check that a row exists in ai_governance_sessions for the workflow.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.