Ports upstream vLLM PRs #54442 and #56802 (issue #54437): never sample from an unmasked row, and key draft-token hand-off snapshots by scheduler step so an alternating PP microbatch cannot overwrite the slot. Co-Authored-By: Hinata <hinata@idihore.id> Signed-off-by: MOVZX <movzx@yahoo.com>