fix(app): stop updates after chat teardown (#1997)

Plain chat views started the history virtualizer even without virtualized rows. Its delayed scroll callback could outlive the mounted view and make the app test job fail during environment teardown.

packages/app/src/agent-stream/strategy-web.tsx

@@ -152,6 +152,7 @@ function WebStreamViewport(props: StreamRenderInput & { isMobileBreakpoint: bool
 
   const rowVirtualizer = useVirtualizer({
     count: segments.historyVirtualized.length,
+    enabled: shouldUseVirtualizer,
     getScrollElement: () => scrollContainerRef.current,
     getItemKey: (index: number) => segments.historyVirtualized[index]?.id ?? index,
     estimateSize: (index: number) => {