NM-288: fix orphan nodes on bulk host deletion and refactor node cleanup

- Extract cleanupNodeReferences() from DeleteNode to handle best-effort
  cleanup of relay, internet gw, failover, nameserver, ACL, and egress refs
- DisassociateAllNodesFromHost now calls cleanupNodeReferences + DeleteNodeByID
  directly, bypassing redundant host-association updates that could fail and
  leave nodes orphaned
- Track failed node deletions instead of unconditionally clearing host.Nodes;
  return error if any nodes couldn't be deleted to prevent host removal
- Fix DeleteNode returning error even when node was successfully deleted in
  the host-not-found path