List distinct session IDs from span attributes.

Returns the distinct values of ag.session.id across spans in the current project, in a windowed, cursor-paginated form. Use this to drive a session-picker UI before drilling into the spans of each session.

The realtime flag controls the cursor field:

• false or unset — paginate by a stable first_active cursor (safe to iterate under heavy write load).
• true — paginate by last_active, reflecting ongoing activity but less stable between pages.

The response includes a windowing cursor; pass it as windowing.next on the next call to continue.