fix: remove unstable key on CodeBlock to prevent flickering during streaming

The original Math.random() key caused React to remount the CodeBlock on
every render. The PR's content-based key (language + content prefix) still
changed every streaming token, causing the same flickering. Removing the
key entirely lets React use positional identity, which is stable across
re-renders while content streams in.

Closes CopilotKit#2669

Author: jpr5

packages/react-ui/src/components/chat/Markdown.tsx

@@ -59,7 +59,6 @@ const defaultComponents: Components = {
 
     return (
       <CodeBlock
-        key={Math.random()}
         language={(match && match[1]) || ""}
         value={String(children).replace(/\n$/, "")}
         {...props}