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/suite/compact.rs

@@ -28,6 +28,7 @@ use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
 use core_test_support::responses::ev_completed_with_tokens;
 use core_test_support::responses::ev_function_call;
+use core_test_support::responses::get_responses_requests;
 use core_test_support::responses::mount_compact_json_once;
 use core_test_support::responses::mount_sse_once;
 use core_test_support::responses::mount_sse_once_match;
@@ -135,7 +136,10 @@ async fn summarize_context_three_requests_and_instructions() {
     config.model_provider = model_provider;
     set_test_compact_prompt(&mut config);
     config.model_auto_compact_token_limit = Some(200_000);
-    let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+    let conversation_manager = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    );
     let NewConversation {
         conversation: codex,
         session_configured,
@@ -329,7 +333,10 @@ async fn manual_compact_uses_custom_prompt() {
     config.model_provider = model_provider;
     config.compact_prompt = Some(custom_prompt.to_string());
 
-    let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+    let conversation_manager = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    );
     let codex = conversation_manager
         .new_conversation(config)
         .await
@@ -344,7 +351,7 @@ async fn manual_compact_uses_custom_prompt() {
     assert_eq!(message, COMPACT_WARNING_MESSAGE);
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
-    let requests = server.received_requests().await.expect("collect requests");
+    let requests = get_responses_requests(&server).await;
     let body = requests
         .iter()
         .find_map(|req| req.body_json::<serde_json::Value>().ok())
@@ -409,7 +416,10 @@ async fn manual_compact_emits_api_and_local_token_usage_events() {
     config.model_provider = model_provider;
     set_test_compact_prompt(&mut config);
 
-    let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+    let conversation_manager = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    );
     let NewConversation {
         conversation: codex,
         ..
@@ -570,7 +580,7 @@ async fn multiple_auto_compact_per_task_runs_after_token_limit_hit() {
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
     // collect the requests payloads from the model
-    let requests_payloads = server.received_requests().await.unwrap();
+    let requests_payloads = get_responses_requests(&server).await;
 
     let body = requests_payloads[0]
         .body_json::<serde_json::Value>()
@@ -1050,7 +1060,10 @@ async fn auto_compact_runs_after_token_limit_hit() {
     config.model_provider = model_provider;
     set_test_compact_prompt(&mut config);
     config.model_auto_compact_token_limit = Some(200_000);
-    let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+    let conversation_manager = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    );
     let codex = conversation_manager
         .new_conversation(config)
         .await
@@ -1090,7 +1103,7 @@ async fn auto_compact_runs_after_token_limit_hit() {
 
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
-    let requests = server.received_requests().await.unwrap();
+    let requests = get_responses_requests(&server).await;
     assert_eq!(
         requests.len(),
         5,
@@ -1295,7 +1308,10 @@ async fn auto_compact_persists_rollout_entries() {
     let mut config = load_default_config_for_test(&home);
     config.model_provider = model_provider;
     set_test_compact_prompt(&mut config);
-    let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+    let conversation_manager = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    );
     let NewConversation {
         conversation: codex,
         session_configured,
@@ -1397,11 +1413,14 @@ async fn manual_compact_retries_after_context_window_error() {
     config.model_provider = model_provider;
     set_test_compact_prompt(&mut config);
     config.model_auto_compact_token_limit = Some(200_000);
-    let codex = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"))
-        .new_conversation(config)
-        .await
-        .unwrap()
-        .conversation;
+    let codex = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    )
+    .new_conversation(config)
+    .await
+    .unwrap()
+    .conversation;
 
     codex
         .submit(Op::UserInput {
@@ -1529,11 +1548,14 @@ async fn manual_compact_twice_preserves_latest_user_messages() {
     let mut config = load_default_config_for_test(&home);
     config.model_provider = model_provider;
     set_test_compact_prompt(&mut config);
-    let codex = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"))
-        .new_conversation(config)
-        .await
-        .unwrap()
-        .conversation;
+    let codex = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    )
+    .new_conversation(config)
+    .await
+    .unwrap()
+    .conversation;
 
     codex
         .submit(Op::UserInput {
@@ -1731,7 +1753,10 @@ async fn auto_compact_allows_multiple_attempts_when_interleaved_with_other_turn_
     config.model_provider = model_provider;
     set_test_compact_prompt(&mut config);
     config.model_auto_compact_token_limit = Some(200);
-    let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+    let conversation_manager = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    );
     let codex = conversation_manager
         .new_conversation(config)
         .await
@@ -1771,10 +1796,8 @@ async fn auto_compact_allows_multiple_attempts_when_interleaved_with_other_turn_
         "auto compact should not emit task lifecycle events"
     );
 
-    let request_bodies: Vec<String> = server
-        .received_requests()
-        .await
-        .unwrap()
+    let requests = get_responses_requests(&server).await;
+    let request_bodies: Vec<String> = requests
         .into_iter()
         .map(|request| String::from_utf8(request.body).unwrap_or_default())
         .collect();
@@ -1845,11 +1868,14 @@ async fn auto_compact_triggers_after_function_call_over_95_percent_usage() {
     config.model_context_window = Some(context_window);
     config.model_auto_compact_token_limit = Some(limit);
 
-    let codex = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"))
-        .new_conversation(config)
-        .await
-        .unwrap()
-        .conversation;
+    let codex = ConversationManager::with_models_provider(
+        CodexAuth::from_api_key("dummy"),
+        config.model_provider.clone(),
+    )
+    .new_conversation(config)
+    .await
+    .unwrap()
+    .conversation;
 
     codex
         .submit(Op::UserInput {