#802 jumper_client.get_run_checkpoint fallback without tenant_id

Issue E: jumper_client.py get_run_checkpoint had fallback code path that allowed checkpoint queries WITHOUT tenant_id filter. This violated defense-in-depth multi-tenant security. Fix: Made tenant_id a required parameter (changed from str | None to str) and removed the fallback code path entirely. All callers already pass tenant_id. Files modified: engine/jumper_client.py (lines 1392-1415)