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

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Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>