Fixed flaky unit test (#5654)

This PR fixes a test that is sporadically failing in CI.

The problem is that two unit tests (the older `login_and_cancel_chatgpt`
and a recently added
`login_chatgpt_includes_forced_workspace_query_param`) exercise code
paths that start the login server. The server binds to a hard-coded
localhost port number, so attempts to start more than one server at the
same time will fail. If these two tests happen to run concurrently, one
of them will fail.

To fix this, I've added a simple mutex. We can use this same mutex for
future tests that use the same pattern.

codex-rs/Cargo.lock

@@ -853,6 +853,7 @@ dependencies = [
  "pretty_assertions",
  "serde",
  "serde_json",
+ "serial_test",
  "tempfile",
  "tokio",
  "toml",

codex-rs/app-server/Cargo.toml

@@ -47,6 +47,7 @@ base64 = { workspace = true }
 core_test_support = { workspace = true }
 os_info = { workspace = true }
 pretty_assertions = { workspace = true }
+serial_test = { workspace = true }
 tempfile = { workspace = true }
 toml = { workspace = true }
 wiremock = { workspace = true }

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

@@ -13,6 +13,7 @@ use codex_app_server_protocol::LoginChatGptResponse;
 use codex_app_server_protocol::LogoutChatGptResponse;
 use codex_app_server_protocol::RequestId;
 use codex_login::login_with_api_key;
+use serial_test::serial;
 use tempfile::TempDir;
 use tokio::time::timeout;
 
@@ -94,6 +95,8 @@ async fn logout_chatgpt_removes_auth() {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+// Serialize tests that launch the login server since it binds to a fixed port.
+#[serial(login_port)]
 async fn login_and_cancel_chatgpt() {
     let codex_home = TempDir::new().unwrap_or_else(|e| panic!("create tempdir: {e}"));
     create_config_toml(codex_home.path()).unwrap_or_else(|err| panic!("write config.toml: {err}"));
@@ -208,6 +211,8 @@ async fn login_chatgpt_rejected_when_forced_api() {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+// Serialize tests that launch the login server since it binds to a fixed port.
+#[serial(login_port)]
 async fn login_chatgpt_includes_forced_workspace_query_param() {
     let codex_home = TempDir::new().unwrap_or_else(|e| panic!("create tempdir: {e}"));
     create_config_toml_forced_workspace(codex_home.path(), "ws-forced")