Decide which panel should be active for a dock based on ordering panels (#22346)

This means that `workspace::ToggleRightDock` will open the assistant if
no right-dock panel has been manually activated, instead of the chat as
before. Also cleans up the `active_panel_index` logic a bit.

cc @nathansobo 

Release Notes:

- Make `workspace::ToggleRightDock` open the assistant panel if no
right-dock panel has yet been activated

crates/assistant2/src/assistant_panel.rs

@@ -286,6 +286,10 @@ impl Panel for AssistantPanel {
     fn toggle_action(&self) -> Box<dyn Action> {
         Box::new(ToggleFocus)
     }
+
+    fn activation_priority(&self) -> u32 {
+        3
+    }
 }
 
 impl AssistantPanel {