v8.4.19: Platform training actually syncs now v8.4.19

Shipped a proper fix for Ultralytics Platform training where model_id now gets passed consistently during uploads and events (turns out scattering state across the trainer was bad, who knew). Refactored the whole callback flow into a single trainer.platform context so cancellation, logging, and session continuity actually work. Also cleaned up SAM predictions and tightened YOLO26 docs because we ship complete things.