Closes https://github.com/zed-industries/zed/issues/23023

Fixes terminal pane button opening two terminals on click.

The culprit is in

https://github.com/zed-industries/zed/blob/61115bd04792d87a540558753cd01a6368baba2a/crates/workspace/src/workspace.rs#L2412-L2417

* We cannot get any panel by index from the Dock, only an active one
* Both `dock.activate_panel(panel_index, cx);` and `dock.set_open(true,
cx);` do `active_panel.panel.set_active(true, cx);`

So, follow other pane's impls that have `active: bool` property for this
case, e.g.
https://github.com/zed-industries/zed/blob/3ec52d8451dd1178c4d036da3a71c827df422f4b/crates/assistant/src/inline_assistant.rs#L2687

Release Notes:

- Fixed terminal pane button opening two terminals on click