feat: honor /etc/codex/config.toml (#8461)

This adds logic to load `/etc/codex/config.toml` and associate it with
`ConfigLayerSource::System` on UNIX. I refactored the code so it shares
logic with the creation of the `ConfigLayerSource::User` layer.

codex-rs/app-server/tests/suite/v2/config_rpc.rs

@@ -18,6 +18,7 @@ use codex_app_server_protocol::RequestId;
 use codex_app_server_protocol::SandboxMode;
 use codex_app_server_protocol::ToolsV2;
 use codex_app_server_protocol::WriteStatus;
+use codex_core::config_loader::SYSTEM_CONFIG_TOML_FILE_UNIX;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use pretty_assertions::assert_eq;
 use serde_json::json;
@@ -73,8 +74,7 @@ sandbox_mode = "workspace-write"
         }
     );
     let layers = layers.expect("layers present");
-    assert_eq!(layers.len(), 1);
-    assert_eq!(layers[0].name, ConfigLayerSource::User { file: user_file });
+    assert_layers_user_then_optional_system(&layers, user_file)?;
 
     Ok(())
 }
@@ -136,8 +136,7 @@ view_image = false
     );
 
     let layers = layers.expect("layers present");
-    assert_eq!(layers.len(), 1);
-    assert_eq!(layers[0].name, ConfigLayerSource::User { file: user_file });
+    assert_layers_user_then_optional_system(&layers, user_file)?;
 
     Ok(())
 }
@@ -257,12 +256,7 @@ writable_roots = [{}]
     );
 
     let layers = layers.expect("layers present");
-    assert_eq!(layers.len(), 2);
-    assert_eq!(
-        layers[0].name,
-        ConfigLayerSource::LegacyManagedConfigTomlFromFile { file: managed_file }
-    );
-    assert_eq!(layers[1].name, ConfigLayerSource::User { file: user_file });
+    assert_layers_managed_user_then_optional_system(&layers, managed_file, user_file)?;
 
     Ok(())
 }
@@ -433,3 +427,50 @@ async fn config_batch_write_applies_multiple_edits() -> Result<()> {
 
     Ok(())
 }
+
+fn assert_layers_user_then_optional_system(
+    layers: &[codex_app_server_protocol::ConfigLayer],
+    user_file: AbsolutePathBuf,
+) -> Result<()> {
+    if cfg!(unix) {
+        let system_file = AbsolutePathBuf::from_absolute_path(SYSTEM_CONFIG_TOML_FILE_UNIX)?;
+        assert_eq!(layers.len(), 2);
+        assert_eq!(layers[0].name, ConfigLayerSource::User { file: user_file });
+        assert_eq!(
+            layers[1].name,
+            ConfigLayerSource::System { file: system_file }
+        );
+    } else {
+        assert_eq!(layers.len(), 1);
+        assert_eq!(layers[0].name, ConfigLayerSource::User { file: user_file });
+    }
+    Ok(())
+}
+
+fn assert_layers_managed_user_then_optional_system(
+    layers: &[codex_app_server_protocol::ConfigLayer],
+    managed_file: AbsolutePathBuf,
+    user_file: AbsolutePathBuf,
+) -> Result<()> {
+    if cfg!(unix) {
+        let system_file = AbsolutePathBuf::from_absolute_path(SYSTEM_CONFIG_TOML_FILE_UNIX)?;
+        assert_eq!(layers.len(), 3);
+        assert_eq!(
+            layers[0].name,
+            ConfigLayerSource::LegacyManagedConfigTomlFromFile { file: managed_file }
+        );
+        assert_eq!(layers[1].name, ConfigLayerSource::User { file: user_file });
+        assert_eq!(
+            layers[2].name,
+            ConfigLayerSource::System { file: system_file }
+        );
+    } else {
+        assert_eq!(layers.len(), 2);
+        assert_eq!(
+            layers[0].name,
+            ConfigLayerSource::LegacyManagedConfigTomlFromFile { file: managed_file }
+        );
+        assert_eq!(layers[1].name, ConfigLayerSource::User { file: user_file });
+    }
+    Ok(())
+}