[FEATURE]: Add copy button to code blocks in user messages #20943
Description
Feature hasn't been suggested before.
- I have verified this feature I'm about to request hasn't been suggested before.
Describe the enhancement you want to request
After working on #20942, I noticed that code blocks in user messages don't
have a copy button, while code blocks in AI responses already do.
I'd like to add a copy button to the top-right corner of the language label
bar in user message code blocks, to keep the experience consistent.