🐛 fix(playground): ensure proper streaming updates & safeguard message handling

Summary
This commit addresses two critical issues affecting the real-time chat experience in the Playground:

1. Optimized re-rendering of reasoning content
   • Added `reasoningContent` to the comparison function of `OptimizedMessageContent` (`web/src/components/playground/OptimizedComponents.js`).
   • Ensures the component re-renders while reasoning text streams, resolving the bug where only the first characters (“好,”) were shown until the stream finished.

2. Defensive checks for SSE message updates
   • Added early-return guards in `streamMessageUpdate` (`web/src/hooks/useApiRequest.js`).
   • Skips updates when `lastMessage` is undefined or the last message isn’t from the assistant, preventing `TypeError: Cannot read properties of undefined (reading 'status')` during rapid SSE responses.

Impact
• Real-time reasoning content now appears progressively, enhancing user feedback.
• Eliminates runtime crashes caused by undefined message references, improving overall stability.

web/src/components/playground/OptimizedComponents.js

@@ -12,6 +12,7 @@ export const OptimizedMessageContent = React.memo(MessageContent, (prevProps, ne
     prevProps.message.content === nextProps.message.content &&
     prevProps.message.status === nextProps.message.status &&
     prevProps.message.role === nextProps.message.role &&
+    prevProps.message.reasoningContent === nextProps.message.reasoningContent &&
     prevProps.message.isReasoningExpanded === nextProps.message.isReasoningExpanded &&
     prevProps.isEditing === nextProps.isEditing &&
     prevProps.editValue === nextProps.editValue &&