Stabilize app-server notify initialize test (#13939)

## What changed
- This PR changes only the flaky test setup for
`turn_start_notify_payload_includes_initialize_client_name`.
- Instead of shelling out to `python3` to write the notify payload, the
test uses the first-party `codex-app-server-test-notify-capture` helper.
- The helper writes `notify.json` atomically and the test waits for the
file to exist before reading it.

## Why this fixes the flake
- The old test depended on an external Python interpreter being present
and behaving consistently on every CI runner.
- It also raced the file write: the test could observe the path before
the payload had been fully written, which produced partial reads and
intermittent assertion failures.
- Moving the write into a repo-owned helper removes the external
dependency, and atomic write-plus-wait makes the handoff deterministic.

## Scope
- Test-only change.

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

@@ -193,13 +193,13 @@ async fn turn_start_notify_payload_includes_initialize_client_name() -> Result<(
     let server = create_mock_responses_server_sequence_unchecked(responses).await;
     let codex_home = TempDir::new()?;
     let notify_file = codex_home.path().join("notify.json");
-    let notify_capture = cargo_bin("test_notify_capture")?;
+    let notify_capture = cargo_bin("codex-app-server-test-notify-capture")?;
     let notify_capture = notify_capture
         .to_str()
         .expect("notify capture path should be valid UTF-8");
-    let notify_file = notify_file
+    let notify_file_str = notify_file
         .to_str()
-        .expect("notify output path should be valid UTF-8");
+        .expect("notify file path should be valid UTF-8");
     create_config_toml_with_extra(
         codex_home.path(),
         &server.uri(),
@@ -207,7 +207,7 @@ async fn turn_start_notify_payload_includes_initialize_client_name() -> Result<(
         &format!(
             "notify = [{}, {}]",
             toml_basic_string(notify_capture),
-            toml_basic_string(notify_file)
+            toml_basic_string(notify_file_str)
         ),
     )?;
 
@@ -255,9 +255,8 @@ async fn turn_start_notify_payload_includes_initialize_client_name() -> Result<(
     )
     .await??;
 
-    let notify_file = Path::new(notify_file);
-    fs_wait::wait_for_path_exists(notify_file, Duration::from_secs(5)).await?;
-    let payload_raw = tokio::fs::read_to_string(notify_file).await?;
+    fs_wait::wait_for_path_exists(&notify_file, Duration::from_secs(5)).await?;
+    let payload_raw = tokio::fs::read_to_string(&notify_file).await?;
     let payload: Value = serde_json::from_str(&payload_raw)?;
     assert_eq!(payload["client"], "xcode");
 
@@ -292,6 +291,9 @@ model_provider = "mock_provider"
 
 {extra}
 
+[features]
+shell_snapshot = false
+
 [model_providers.mock_provider]
 name = "Mock provider for test"
 base_url = "{server_uri}/v1"