fix(ui): prevent empty div flicker between tool transitions

The text wrapper div was rendering when `isStreaming` was true but
`text` was empty, causing a brief empty flex container to appear
between tool executions. Changed condition from `(text || isStreaming)`
to `text` only, so the div only renders when there's actual content.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/ui/src/components/message-list.tsx

@@ -119,8 +119,8 @@ export const MessageList = memo(function MessageList({
               <ThinkingItem thinking={thinking} isStreaming={isStreaming} />
             )}
 
-            {/* Render text content (if any) */}
-            {(text || isStreaming) && (
+            {/* Render text content (if any) - require actual text, not just streaming state */}
+            {text && (
               <div
                 className={cn(
                   "flex items-center gap-1.5",