Fix Qwen3 MoE double-reduce when DP attention + EP + reduce_scatterv (#23729)#23731

ByronHsu merged 1 commit intosgl-project:mainsgl-project/sglang:mainfrom

ByronHsu:fix/qwen3-moe-dp-reduce-scatterv-guardByronHsu/sglang:fix/qwen3-moe-dp-reduce-scatterv-guardCopy head branch name to clipboard