fix(ui): stabilize streaming message/tool rendering

Avoid remounting message blocks on part updates so tool call UI state persists. Render tool/message content from store and stabilize tool output scrolling during streaming.

packages/ui/src/components/tool-call.tsx

@@ -235,12 +235,16 @@ export default function ToolCall(props: ToolCallProps) {
     requestAnimationFrame(() => {
       restoreScrollPosition(autoScroll())
       if (!expanded()) return
-      scheduleAnchorScroll()
+      scheduleAnchorScroll(true)
     })
   }
 
   const initializeScrollContainer = (element: HTMLDivElement | null | undefined) => {
-    scrollContainerRef = element || undefined
+    const next = element || undefined
+    if (next === scrollContainerRef) {
+      return
+    }
+    scrollContainerRef = next
     setScrollContainer(scrollContainerRef)
     if (scrollContainerRef) {
       restoreScrollPosition(autoScroll())
@@ -593,7 +597,7 @@ export default function ToolCall(props: ToolCallProps) {
       return
     }
     previousPartVersion = version
-    scheduleAnchorScroll()
+    scheduleAnchorScroll(true)
   })
 
   createEffect(() => {