Wire up AssistantPanelDelegate based on assistant2 feature flag (#23444)

This PR adjusts how the `AssistantPanelDelegate` global is set to be
based on the state of the feature flag.

This should prevent `assistant` and `assistant2` from potentially
clobbering each other.

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -28,7 +28,7 @@ use prompt_library::{PromptBuilder, PromptLoadingParams};
 use settings::Settings as _;
 use util::ResultExt;
 
-pub use crate::assistant_panel::AssistantPanel;
+pub use crate::assistant_panel::{AssistantPanel, ConcreteAssistantPanelDelegate};
 pub use crate::inline_assistant::InlineAssistant;
 
 actions!(

crates/assistant2/src/assistant_panel.rs

@@ -34,7 +34,6 @@ use crate::thread_store::ThreadStore;
 use crate::{NewPromptEditor, NewThread, OpenHistory, OpenPromptEditorHistory};
 
 pub fn init(cx: &mut AppContext) {
-    <dyn AssistantPanelDelegate>::set_global(Arc::new(ConcreteAssistantPanelDelegate), cx);
     cx.observe_new_views(
         |workspace: &mut Workspace, _cx: &mut ViewContext<Workspace>| {
             workspace
@@ -811,7 +810,7 @@ impl Render for AssistantPanel {
     }
 }
 
-struct ConcreteAssistantPanelDelegate;
+pub struct ConcreteAssistantPanelDelegate;
 
 impl AssistantPanelDelegate for ConcreteAssistantPanelDelegate {
     fn active_context_editor(