Vouch Request: Sakshamm-Goyal — focused fix for #4326 #4365

I would like to contribute a focused TypeScript fix for #4326 (the watch-mode chat transport reuses a useChat stream across turns, duplicating the previous assistant message and leaving status non-ready).\n\nI will keep the PR limited to this one issue, first reproduce it with a regression test, then add the smallest transport-level fix and repository-prescribed validation. I will not mix unrelated changes.\n\nRelevant recent OSS work: DataHub PR #18587 (TypeScript/React/GraphQL, all source-validation workflows green) and PostHog PR #72484.\n\nPlease vouch me if this scope and contribution process are acceptable. Thank you.