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/renderers/task.tsx

@@ -176,7 +176,7 @@ export const taskRenderer: ToolRenderer = {
             <div class="tool-call-task-section-body">
               <div
                 class="message-text tool-call-markdown tool-call-task-container"
-                ref={(element) => scrollHelpers?.registerContainer(element)}
+                ref={scrollHelpers?.registerContainer}
                 onScroll={
                   scrollHelpers ? (event) => scrollHelpers.handleScroll(event as Event & { currentTarget: HTMLDivElement }) : undefined
                 }