agent: Make AgentSettings::default_model optional (#33695)

It's already effectively optional and the the old default of gpt-4
doesn't really get used in practice

Release Notes:

- N/A

crates/agent_ui/src/agent_ui.rs

@@ -211,7 +211,7 @@ fn update_active_language_model_from_settings(cx: &mut App) {
         }
     }
 
-    let default = to_selected_model(&settings.default_model);
+    let default = settings.default_model.as_ref().map(to_selected_model);
     let inline_assistant = settings
         .inline_assistant_model
         .as_ref()
@@ -231,7 +231,7 @@ fn update_active_language_model_from_settings(cx: &mut App) {
         .collect::<Vec<_>>();
 
     LanguageModelRegistry::global(cx).update(cx, |registry, cx| {
-        registry.select_default_model(Some(&default), cx);
+        registry.select_default_model(default.as_ref(), cx);
         registry.select_inline_assistant_model(inline_assistant.as_ref(), cx);
         registry.select_commit_message_model(commit_message.as_ref(), cx);
         registry.select_thread_summary_model(thread_summary.as_ref(), cx);