- Custom status now uses history events (CustomStatusUpdated) instead of
  ExecutionMetadata.custom_status (removed upstream)
- ack_orchestration_item scans history_delta for last CustomStatusUpdated event
- Added short_poll_threshold() override (500ms) for ProviderFactory
- Wired test_orphan_activity_after_instance_force_deletion validation test
- Added initial_custom_status: None to OrchestrationStarted test literals

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>