Fixes #51048

The "Copy Command" button uses `.visible_on_hover(group)` from GPUI to
only appear when the user hovers over its parent container. In
`render_collapsible_command` (used to render the code blocks for
terminal tool calls like "Run Command"), the parent `v_flex()` container
was missing the `.group()` assignment. This caused the copy button to
never become visible, which became apparent when an agent session was
restored from history.

This commit adds `.group(group.clone())` to the root `v_flex()`
container in `render_collapsible_command` to restore the hover
visibility for the "Copy Command" button.

Video :
[Screencast from 2026-03-10
18-06-57.webm](https://github.com/user-attachments/assets/ae931ac3-c7f1-4044-a3d8-a91a93d9c3bb)
[Screencast from 2026-03-10
18-06-01.webm](https://github.com/user-attachments/assets/5ddb8085-bafe-4e9a-bb02-74e3d860ae1a)

Release Notes:

- Agent: Fixed an issue where the "Copy Command" button would not appear
on hover for terminal tool calls.