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/core/src/config/service.rs

@@ -778,15 +778,41 @@ remote_compaction = true
             },
         );
         let layers = response.layers.expect("layers present");
-        assert_eq!(layers.len(), 2, "expected two layers");
-        assert_eq!(
-            layers.first().unwrap().name,
-            ConfigLayerSource::LegacyManagedConfigTomlFromFile { file: managed_file }
-        );
-        assert_eq!(
-            layers.get(1).unwrap().name,
-            ConfigLayerSource::User { file: user_file }
-        );
+        if cfg!(unix) {
+            let system_file = AbsolutePathBuf::from_absolute_path(
+                crate::config_loader::SYSTEM_CONFIG_TOML_FILE_UNIX,
+            )
+            .expect("system file");
+            assert_eq!(layers.len(), 3, "expected three layers on unix");
+            assert_eq!(
+                layers.first().unwrap().name,
+                ConfigLayerSource::LegacyManagedConfigTomlFromFile {
+                    file: managed_file.clone()
+                }
+            );
+            assert_eq!(
+                layers.get(1).unwrap().name,
+                ConfigLayerSource::User {
+                    file: user_file.clone()
+                }
+            );
+            assert_eq!(
+                layers.get(2).unwrap().name,
+                ConfigLayerSource::System { file: system_file }
+            );
+        } else {
+            assert_eq!(layers.len(), 2, "expected two layers");
+            assert_eq!(
+                layers.first().unwrap().name,
+                ConfigLayerSource::LegacyManagedConfigTomlFromFile {
+                    file: managed_file.clone()
+                }
+            );
+            assert_eq!(
+                layers.get(1).unwrap().name,
+                ConfigLayerSource::User { file: user_file }
+            );
+        }
     }
 
     #[tokio::test]