feat: clean config loading and config api (#7924)

Check the README of the `config_loader` for details

codex-rs/core/src/config_loader/tests.rs

@@ -0,0 +1,138 @@
+use super::LoaderOverrides;
+use super::load_config_layers_state;
+use crate::config::CONFIG_TOML_FILE;
+use tempfile::tempdir;
+use toml::Value as TomlValue;
+
+#[tokio::test]
+async fn merges_managed_config_layer_on_top() {
+    let tmp = tempdir().expect("tempdir");
+    let managed_path = tmp.path().join("managed_config.toml");
+
+    std::fs::write(
+        tmp.path().join(CONFIG_TOML_FILE),
+        r#"foo = 1
+
+[nested]
+value = "base"
+"#,
+    )
+    .expect("write base");
+    std::fs::write(
+        &managed_path,
+        r#"foo = 2
+
+[nested]
+value = "managed_config"
+extra = true
+"#,
+    )
+    .expect("write managed config");
+
+    let overrides = LoaderOverrides {
+        managed_config_path: Some(managed_path),
+        #[cfg(target_os = "macos")]
+        managed_preferences_base64: None,
+    };
+
+    let state = load_config_layers_state(tmp.path(), &[] as &[(String, TomlValue)], overrides)
+        .await
+        .expect("load config");
+    let loaded = state.effective_config();
+    let table = loaded.as_table().expect("top-level table expected");
+
+    assert_eq!(table.get("foo"), Some(&TomlValue::Integer(2)));
+    let nested = table
+        .get("nested")
+        .and_then(|v| v.as_table())
+        .expect("nested");
+    assert_eq!(
+        nested.get("value"),
+        Some(&TomlValue::String("managed_config".to_string()))
+    );
+    assert_eq!(nested.get("extra"), Some(&TomlValue::Boolean(true)));
+}
+
+#[tokio::test]
+async fn returns_empty_when_all_layers_missing() {
+    let tmp = tempdir().expect("tempdir");
+    let managed_path = tmp.path().join("managed_config.toml");
+    let overrides = LoaderOverrides {
+        managed_config_path: Some(managed_path),
+        #[cfg(target_os = "macos")]
+        managed_preferences_base64: None,
+    };
+
+    let layers = load_config_layers_state(tmp.path(), &[] as &[(String, TomlValue)], overrides)
+        .await
+        .expect("load layers");
+    let base_table = layers.user.config.as_table().expect("base table expected");
+    assert!(
+        base_table.is_empty(),
+        "expected empty base layer when configs missing"
+    );
+    assert!(
+        layers.system.is_none(),
+        "managed config layer should be absent when file missing"
+    );
+
+    #[cfg(not(target_os = "macos"))]
+    {
+        let effective = layers.effective_config();
+        let table = effective.as_table().expect("top-level table expected");
+        assert!(
+            table.is_empty(),
+            "expected empty table when configs missing"
+        );
+    }
+}
+
+#[cfg(target_os = "macos")]
+#[tokio::test]
+async fn managed_preferences_take_highest_precedence() {
+    use base64::Engine;
+
+    let managed_payload = r#"
+[nested]
+value = "managed"
+flag = false
+"#;
+    let encoded = base64::prelude::BASE64_STANDARD.encode(managed_payload.as_bytes());
+    let tmp = tempdir().expect("tempdir");
+    let managed_path = tmp.path().join("managed_config.toml");
+
+    std::fs::write(
+        tmp.path().join(CONFIG_TOML_FILE),
+        r#"[nested]
+value = "base"
+"#,
+    )
+    .expect("write base");
+    std::fs::write(
+        &managed_path,
+        r#"[nested]
+value = "managed_config"
+flag = true
+"#,
+    )
+    .expect("write managed config");
+
+    let overrides = LoaderOverrides {
+        managed_config_path: Some(managed_path),
+        managed_preferences_base64: Some(encoded),
+    };
+
+    let state = load_config_layers_state(tmp.path(), &[] as &[(String, TomlValue)], overrides)
+        .await
+        .expect("load config");
+    let loaded = state.effective_config();
+    let nested = loaded
+        .get("nested")
+        .and_then(|v| v.as_table())
+        .expect("nested table");
+    assert_eq!(
+        nested.get("value"),
+        Some(&TomlValue::String("managed".to_string()))
+    );
+    assert_eq!(nested.get("flag"), Some(&TomlValue::Boolean(false)));
+}