fix: added test helpers for platform-specific paths (#7954)

This addresses post-merge feedback from
https://github.com/openai/codex/pull/7856.

codex-rs/app-server/tests/suite/config.rs

@@ -1,5 +1,6 @@
 use anyhow::Result;
 use app_test_support::McpProcess;
+use app_test_support::test_tmp_path;
 use app_test_support::to_response;
 use codex_app_server_protocol::GetUserSavedConfigResponse;
 use codex_app_server_protocol::JSONRPCResponse;
@@ -14,7 +15,6 @@ use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::openai_models::ReasoningEffort;
-use codex_utils_absolute_path::AbsolutePathBuf;
 use pretty_assertions::assert_eq;
 use std::collections::HashMap;
 use std::path::Path;
@@ -24,11 +24,7 @@ use tokio::time::timeout;
 const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
 
 fn create_config_toml(codex_home: &Path) -> std::io::Result<()> {
-    let writable_root = if cfg!(windows) {
-        r"C:\Users\codex\AppData\Local\Temp"
-    } else {
-        "/tmp"
-    };
+    let writable_root = test_tmp_path();
     let config_toml = codex_home.join("config.toml");
     std::fs::write(
         config_toml,
@@ -84,12 +80,7 @@ async fn get_config_toml_parses_all_fields() -> Result<()> {
     .await??;
 
     let config: GetUserSavedConfigResponse = to_response(resp)?;
-    let writable_root = if cfg!(windows) {
-        r"C:\Users\codex\AppData\Local\Temp"
-    } else {
-        "/tmp"
-    };
-    let writable_root = AbsolutePathBuf::from_absolute_path(writable_root)?;
+    let writable_root = test_tmp_path();
     let expected = GetUserSavedConfigResponse {
         config: UserSavedConfig {
             approval_policy: Some(AskForApproval::OnRequest),