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/core/tests/common/test_codex.rs

@@ -23,6 +23,7 @@ use tempfile::TempDir;
 use wiremock::MockServer;
 
 use crate::load_default_config_for_test;
+use crate::responses::get_responses_request_bodies;
 use crate::responses::start_mock_server;
 use crate::wait_for_event;
 
@@ -69,7 +70,7 @@ impl TestCodexBuilder {
     pub fn with_model(self, model: &str) -> Self {
         let new_model = model.to_string();
         self.with_config(move |config| {
-            config.model = new_model.clone();
+            config.model = Some(new_model.clone());
         })
     }
 
@@ -96,7 +97,8 @@ impl TestCodexBuilder {
         let (config, cwd) = self.prepare_config(server, &home).await?;
 
         let auth = self.auth.clone();
-        let conversation_manager = ConversationManager::with_auth(auth.clone());
+        let conversation_manager =
+            ConversationManager::with_models_provider(auth.clone(), config.model_provider.clone());
 
         let new_conversation = match resume_from {
             Some(path) => {
@@ -272,13 +274,7 @@ impl TestCodexHarness {
     }
 
     pub async fn request_bodies(&self) -> Vec<Value> {
-        self.server
-            .received_requests()
-            .await
-            .expect("requests")
-            .into_iter()
-            .map(|req| serde_json::from_slice(&req.body).expect("request body json"))
-            .collect()
+        get_responses_request_bodies(&self.server).await
     }
 
     pub async fn function_call_output_value(&self, call_id: &str) -> Value {