Python: DevUI Fix Serialization, Timestamp and Other Issues (#1584)

* refactor(devui): adopt standard OpenAI lifecycle events for agents and workflows

- Replace custom workflow events with OpenAI Responses API standard lifecycle events
- Add AgentStartedEvent, AgentCompletedEvent, AgentFailedEvent for clean separation
- Implement ExecutorActionItem for workflow executor tracking
- Convert informational events to trace events to reduce noise
- Update README mapper table with comprehensive event mappings
- Maintain full backward compatibility with legacy events

* fix(devui): resolve timestamp overwriting and Content serialization errors

- Fix tool call timestamps being overwritten on each render (#1483)
- Add recursive Content serialization to handle ChatMessage and nested objects (#1548)
- Implement proper MCP tool cleanup on server shutdown
- Add timestamp field to function_result.complete events
- Enhance credential and client resource cleanup

Fixes #1483, #1548
Partial improvements for #1476

python/packages/devui/frontend/src/components/ui/markdown-renderer.tsx

@@ -18,7 +18,7 @@
  * - Horizontal rules (---)
  */
 
-import React, { useState } from "react";
+import React, { useState, useRef, useEffect } from "react";
 
 interface MarkdownRendererProps {
   content: string;
@@ -35,10 +35,10 @@ interface CodeBlockProps {
  */
 function CodeBlock({ code, language }: CodeBlockProps) {
   const [copied, setCopied] = useState(false);
-  const timeoutRef = React.useRef<NodeJS.Timeout | null>(null);
+  const timeoutRef = useRef<NodeJS.Timeout | null>(null);
 
   // Cleanup timeout on unmount
-  React.useEffect(() => {
+  useEffect(() => {
     return () => {
       if (timeoutRef.current) {
         clearTimeout(timeoutRef.current);