Draft: feat: add start_agent_platform_session event

Relates to issue #560731 (closed)

Changes

• Add new internal event start_agent_platform_session to track GitLab Duo Agent platform session starts
• Include flow_type as label and session_id as value for analytics tracking
• Add event to AI tracking system for instance-level analytics
• Update existing cleanup_stuck_agent_platform_session event to include flow_type label