Improve model selection in the assistant (#12472)

https://github.com/zed-industries/zed/assets/482957/3b017850-b7b6-457a-9b2f-324d5533442e


Release Notes:

- Improved the UX for selecting a model in the assistant panel. You can
now switch model using just the keyboard by pressing `alt-m`. Also, when
switching models via the UI, settings will now be updated automatically.

crates/assistant/src/completion_provider/open_ai.rs

@@ -11,6 +11,7 @@ use open_ai::{stream_completion, Request, RequestMessage, Role as OpenAiRole};
 use settings::Settings;
 use std::time::Duration;
 use std::{env, sync::Arc};
+use strum::IntoEnumIterator;
 use theme::ThemeSettings;
 use ui::prelude::*;
 use util::ResultExt;
@@ -18,7 +19,7 @@ use util::ResultExt;
 pub struct OpenAiCompletionProvider {
     api_key: Option<String>,
     api_url: String,
-    default_model: OpenAiModel,
+    model: OpenAiModel,
     http_client: Arc<dyn HttpClient>,
     low_speed_timeout: Option<Duration>,
     settings_version: usize,
@@ -26,7 +27,7 @@ pub struct OpenAiCompletionProvider {
 
 impl OpenAiCompletionProvider {
     pub fn new(
-        default_model: OpenAiModel,
+        model: OpenAiModel,
         api_url: String,
         http_client: Arc<dyn HttpClient>,
         low_speed_timeout: Option<Duration>,
@@ -35,7 +36,7 @@ impl OpenAiCompletionProvider {
         Self {
             api_key: None,
             api_url,
-            default_model,
+            model,
             http_client,
             low_speed_timeout,
             settings_version,
@@ -44,17 +45,21 @@ impl OpenAiCompletionProvider {
 
     pub fn update(
         &mut self,
-        default_model: OpenAiModel,
+        model: OpenAiModel,
         api_url: String,
         low_speed_timeout: Option<Duration>,
         settings_version: usize,
     ) {
-        self.default_model = default_model;
+        self.model = model;
         self.api_url = api_url;
         self.low_speed_timeout = low_speed_timeout;
         self.settings_version = settings_version;
     }
 
+    pub fn available_models(&self) -> impl Iterator<Item = OpenAiModel> {
+        OpenAiModel::iter()
+    }
+
     pub fn settings_version(&self) -> usize {
         self.settings_version
     }
@@ -104,8 +109,8 @@ impl OpenAiCompletionProvider {
             .into()
     }
 
-    pub fn default_model(&self) -> OpenAiModel {
-        self.default_model.clone()
+    pub fn model(&self) -> OpenAiModel {
+        self.model.clone()
     }
 
     pub fn count_tokens(
@@ -152,7 +157,7 @@ impl OpenAiCompletionProvider {
     fn to_open_ai_request(&self, request: LanguageModelRequest) -> Request {
         let model = match request.model {
             LanguageModel::OpenAi(model) => model,
-            _ => self.default_model(),
+            _ => self.model(),
         };
 
         Request {