Allow codebase search to be turned on or off within the composer for assistant2 (#11315)

![image](https://github.com/zed-industries/zed/assets/836375/e03d2357-e2e4-49f1-86d6-7593bce13618)


![image](https://github.com/zed-industries/zed/assets/836375/3d769622-82e1-4e6f-bdec-4dce81e43423)


![image](https://github.com/zed-industries/zed/assets/836375/bf79a4ec-1660-47b1-8525-e741575dc5d4)

Release Notes:

- N/A

crates/assistant2/src/assistant2.rs

@@ -23,7 +23,7 @@ use semantic_index::{CloudEmbeddingProvider, ProjectIndex, SemanticIndex};
 use serde::Deserialize;
 use settings::Settings;
 use std::sync::Arc;
-use ui::Composer;
+use ui::{Composer, ProjectIndexButton};
 use util::{paths::EMBEDDINGS_DIR, ResultExt};
 use workspace::{
     dock::{DockPosition, Panel, PanelEvent},
@@ -228,6 +228,7 @@ pub struct AssistantChat {
     list_state: ListState,
     language_registry: Arc<LanguageRegistry>,
     composer_editor: View<Editor>,
+    project_index_button: Option<View<ProjectIndexButton>>,
     user_store: Model<UserStore>,
     next_message_id: MessageId,
     collapsed_messages: HashMap<MessageId, bool>,
@@ -263,6 +264,10 @@ impl AssistantChat {
             },
         );
 
+        let project_index_button = project_index.clone().map(|project_index| {
+            cx.new_view(|cx| ProjectIndexButton::new(project_index, tool_registry.clone(), cx))
+        });
+
         Self {
             model,
             messages: Vec::new(),
@@ -275,6 +280,7 @@ impl AssistantChat {
             list_state,
             user_store,
             language_registry,
+            project_index_button,
             project_index,
             next_message_id: MessageId(0),
             editing_message: None,
@@ -397,7 +403,7 @@ impl AssistantChat {
                     {
                         this.tool_registry.definitions()
                     } else {
-                        &[]
+                        Vec::new()
                     };
                     call_count += 1;
 
@@ -590,7 +596,7 @@ impl AssistantChat {
                         element.child(Composer::new(
                             body.clone(),
                             self.user_store.read(cx).current_user(),
-                            self.tool_registry.clone(),
+                            self.project_index_button.clone(),
                             crate::ui::ModelSelector::new(
                                 cx.view().downgrade(),
                                 self.model.clone(),
@@ -768,7 +774,7 @@ impl Render for AssistantChat {
             .child(Composer::new(
                 self.composer_editor.clone(),
                 self.user_store.read(cx).current_user(),
-                self.tool_registry.clone(),
+                self.project_index_button.clone(),
                 crate::ui::ModelSelector::new(cx.view().downgrade(), self.model.clone())
                     .into_any_element(),
             ))