Fix: prevent streaming message width collapse (#12999)

## Summary
- keep assistant message containers stretched to available width
- avoid width collapse during streaming by allowing flex items to shrink

## Test plan
- not run (not requested)

Fixes #12985

Made with [Cursor](https://cursor.com)

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Liu An <asiro@qq.com>

web/src/components/message-item/index.module.less

@@ -9,6 +9,8 @@
   .messageItemContent {
     display: inline-flex;
     gap: 20px;
+    width: 100%;
+    min-width: 0;
   }
   .messageItemContentReverse {
     flex-direction: row-reverse;

web/src/components/message-item/index.tsx

@@ -124,7 +124,7 @@ const MessageItem = ({
               ></SvgIcon>
             ))}
 
-          <section className="flex gap-2 flex-1 flex-col">
+          <section className="flex min-w-0 gap-2 flex-1 flex-col">
             {isAssistant ? (
               index !== 0 && (
                 <AssistantGroupButton

web/src/components/next-message-item/index.module.less

@@ -10,6 +10,7 @@
     display: inline-flex;
     gap: 20px;
     width: 100%;
+    min-width: 0;
   }
   .messageItemContentReverse {
     flex-direction: row-reverse;