assistant2: Add "Copy code" button to code blocks (#22866)

Here's what it looks like, including the "Copy" hover text in one case:


![screenshot](https://github.com/user-attachments/assets/c8d27205-9650-493d-bd3c-a8c7beb142f9)


Release Notes:

- N/A

---------

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
7d905d0791Richard Feldman committed on 1/9/2025, 2:32:42 PM· committed by GitHubparenta8ef0f2
1 file changedLine totals unavailable