markup: improve code block readability and isolate copy button (#34009)

Fix #33197

Improve the rendering of code blocks in markdown content 
for better readability and UI stability across screen sizes.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/markup/codecopy.ts

@@ -15,6 +15,8 @@ export function initMarkupCodeCopy(elMarkup: HTMLElement): void {
     const btn = makeCodeCopyButton();
     // remove final trailing newline introduced during HTML rendering
     btn.setAttribute('data-clipboard-text', el.textContent.replace(/\r?\n$/, ''));
-    el.after(btn);
+    // we only want to use `.code-block-container` if it exists, no matter `.code-block` exists or not.
+    const btnContainer = el.closest('.code-block-container') ?? el.closest('.code-block');
+    btnContainer.append(btn);
   });
 }