perf: fix O(N^2) accumulation in MessageStream and BetaMessageStream#997

claygeo wants to merge 2 commits intoanthropics:mainanthropics/anthropic-sdk-typescript:mainfrom

claygeo:fix/streaming-on2-accumulationclaygeo/anthropic-sdk-typescript:fix/streaming-on2-accumulationCopy head branch name to clipboard