OpenAI Agents SDK group_id maps to $ai_session_id

Minor changes

• 4bf123e The OpenAI Agents SDK group_id now also maps to $ai_session_id on $ai_trace and span events, so grouped runs show up as sessions in PostHog AI observability. $ai_group_id is still emitted alongside it. — Thanks @marco-g-pm for your first contribution 🎉!