fix(ui): stabilize Markdown code block line layout (#2444)

* fix(ui): stabilize markdown code block layout

* fix(ui): respect text range boundary affinity

* test(ui): cover trailing text range boundaries

packages/ui/src/components/chat/message/MessageBody.tsx

@@ -1541,6 +1541,7 @@ const AssistantMessageBody = React.memo(({
                 const paddingSize = 24;
 
                 wrapper = document.createElement('div');
+                wrapper.setAttribute('data-message-image-export', 'true');
                 wrapper.style.cssText = `
                     padding: ${paddingSize}px;
                     background-color: ${resolvedBackgroundColor};

packages/ui/src/components/chat/message/parts/UserTextPart.tsx

@@ -236,7 +236,9 @@ const UserTextPart: React.FC<UserTextPartProps> = ({ part, messageId, agentMenti
                                  "[&_[data-component='markdown-code']>div]:p-0",
                                  "[&_[data-component='markdown-code']_pre]:inline",
                                  "[&_[data-component='markdown-code']_code]:inline",
-                                 "[&_[data-md-code-line-numbers]]:hidden",
+                                 "[&_[data-md-code-line]]:!inline",
+                                 "[&_[data-md-code-line-number]]:hidden",
+                                 "[&_[data-md-code-line-break]]:!inline",
                              ]
                         )}
                         disableLinkSafety