Load app-server config through ConfigManager (#18870)

## Summary
- Load app-server startup config through `ConfigManager` instead of
direct `ConfigBuilder` calls.
- Move `ConfigManager` constructor-owned state (`cli_overrides`, runtime
feature map, cloud requirements loader) behind internal manager fields.
- Pass `ConfigManager` into `MessageProcessor` directly instead of
reconstructing it from raw args.

## Tests
- `cargo check -p codex-app-server`
- `cargo test -p codex-app-server`
- `just fix -p codex-app-server`
- `just fmt`

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

@@ -39,17 +39,15 @@ pub(crate) struct ConfigManager {
 impl ConfigManager {
     pub(crate) fn new(
         codex_home: PathBuf,
-        cli_overrides: Arc<RwLock<Vec<(String, TomlValue)>>>,
-        runtime_feature_enablement: Arc<RwLock<BTreeMap<String, bool>>>,
+        cli_overrides: Vec<(String, TomlValue)>,
         loader_overrides: LoaderOverrides,
-        cloud_requirements: Arc<RwLock<CloudRequirementsLoader>>,
+        cloud_requirements: CloudRequirementsLoader,
         arg0_paths: Arg0DispatchPaths,
         thread_config_loader: Arc<dyn ThreadConfigLoader>,
     ) -> Self {
         Self::new_with_host_name(
             codex_home,
             cli_overrides,
-            runtime_feature_enablement,
             loader_overrides,
             cloud_requirements,
             arg0_paths,
@@ -61,20 +59,19 @@ impl ConfigManager {
     #[allow(clippy::too_many_arguments)]
     fn new_with_host_name(
         codex_home: PathBuf,
-        cli_overrides: Arc<RwLock<Vec<(String, TomlValue)>>>,
-        runtime_feature_enablement: Arc<RwLock<BTreeMap<String, bool>>>,
+        cli_overrides: Vec<(String, TomlValue)>,
         loader_overrides: LoaderOverrides,
-        cloud_requirements: Arc<RwLock<CloudRequirementsLoader>>,
+        cloud_requirements: CloudRequirementsLoader,
         arg0_paths: Arg0DispatchPaths,
         thread_config_loader: Arc<dyn ThreadConfigLoader>,
         host_name: Option<String>,
     ) -> Self {
         Self {
             codex_home,
-            cli_overrides,
-            runtime_feature_enablement,
+            cli_overrides: Arc::new(RwLock::new(cli_overrides)),
+            runtime_feature_enablement: Arc::new(RwLock::new(BTreeMap::new())),
             loader_overrides,
-            cloud_requirements,
+            cloud_requirements: Arc::new(RwLock::new(cloud_requirements)),
             arg0_paths,
             thread_config_loader,
             host_name,
@@ -148,6 +145,17 @@ impl ConfigManager {
         .await
     }
 
+    pub(crate) async fn load_default_config(&self) -> std::io::Result<Config> {
+        let mut config = Config::load_default_with_cli_overrides_for_codex_home(
+            self.codex_home.clone(),
+            self.current_cli_overrides(),
+        )
+        .await?;
+        self.apply_runtime_feature_enablement(&mut config);
+        self.apply_arg0_paths(&mut config);
+        Ok(config)
+    }
+
     pub(crate) async fn load_with_overrides(
         &self,
         request_overrides: Option<HashMap<String, serde_json::Value>>,
@@ -262,10 +270,9 @@ impl ConfigManager {
     ) -> Self {
         Self::new_with_host_name(
             codex_home,
-            Arc::new(RwLock::new(cli_overrides)),
-            Arc::new(RwLock::new(BTreeMap::new())),
+            cli_overrides,
             loader_overrides,
-            Arc::new(RwLock::new(cloud_requirements)),
+            cloud_requirements,
             Arg0DispatchPaths::default(),
             Arc::new(codex_config::NoopThreadConfigLoader),
             host_name,