make model optional in config (#7769)

- Make Config.model optional and centralize default-selection logic in
ModelsManager, including a default_model helper (with
codex-auto-balanced when available) so sessions now carry an explicit
chosen model separate from the base config.
- Resolve `model` once in `core` and `tui` from config. Then store the
state of it on other structs.
- Move refreshing models to be before resolving the default model

codex-rs/app-server/src/codex_message_processor.rs

@@ -1885,7 +1885,7 @@ impl CodexMessageProcessor {
 
     async fn list_models(&self, request_id: RequestId, params: ModelListParams) {
         let ModelListParams { limit, cursor } = params;
-        let models = supported_models(self.conversation_manager.clone()).await;
+        let models = supported_models(self.conversation_manager.clone(), &self.config).await;
         let total = models.len();
 
         if total == 0 {
@@ -2796,7 +2796,7 @@ impl CodexMessageProcessor {
         })?;
 
         let mut config = self.config.as_ref().clone();
-        config.model = self.config.review_model.clone();
+        config.model = Some(self.config.review_model.clone());
 
         let NewConversation {
             conversation_id,