debugger beta: Autoscroll to recently saved debug scenario when saving a scenario (#31528)

I added a test to this too as one of my first steps of improving
`NewSessionModal`'s test coverage.


Release Notes:

- debugger beta: Select saved debug config when opening debug.json from
`NewSessionModal`

crates/tasks_ui/src/tasks_ui.rs

@@ -270,7 +270,11 @@ pub fn task_contexts(
                 .read(cx)
                 .worktree_for_id(*worktree_id, cx)
                 .map_or(false, |worktree| is_visible_directory(&worktree, cx))
-        });
+        })
+        .or(workspace
+            .visible_worktrees(cx)
+            .next()
+            .map(|tree| tree.read(cx).id()));
 
     let active_editor = active_item.and_then(|item| item.act_as::<Editor>(cx));