app-server: fix Bazel clippy in tracing tests (#18872)

## Why

PR #18431 exposed a Bazel clippy failure in the app-server unit-test
target across Linux, macOS, and Windows. The failing lint was
`clippy::await_holding_invalid_type`: two tracing tests serialized
access to global tracing state by holding a `tokio::sync::MutexGuard`
across awaited test work.

That serialization is still needed because the tests share
process-global tracing setup and exporter state, but it should not
require holding an async mutex guard through the whole test body.

## What changed

- Replaced the bespoke async `tracing_test_guard` helper with
`serial_test` on the two tracing tests that need global tracing
serialization.
- Removed the `#[expect(clippy::await_holding_invalid_type)]`
annotations and the lock guard callsites that Bazel clippy rejected.

## Validation

- `cargo test -p codex-app-server jsonrpc_span`
- `just fix -p codex-app-server`
- `git diff --check`

I also attempted the exact failing Bazel clippy target locally with
BuildBuddy disabled: `bazel --noexperimental_remote_repo_contents_cache
build --config=clippy --bes_backend= --remote_cache=
--experimental_remote_downloader= --
//codex-rs/app-server:app-server-unit-tests-bin`. That run did not reach
clippy because Bazel timed out downloading `libcap-2.27.tar.gz` from
`kernel.org`.

codex-rs/app-server/src/message_processor/tracing_tests.rs

@@ -40,6 +40,7 @@ use opentelemetry_sdk::trace::InMemorySpanExporter;
 use opentelemetry_sdk::trace::SdkTracerProvider;
 use opentelemetry_sdk::trace::SpanData;
 use pretty_assertions::assert_eq;
+use serial_test::serial;
 use std::collections::BTreeMap;
 use std::path::Path;
 use std::sync::Arc;
@@ -101,11 +102,6 @@ fn request_from_client_request(request: ClientRequest) -> JSONRPCRequest {
         .expect("client request should convert to JSON-RPC")
 }
 
-fn tracing_test_guard() -> &'static tokio::sync::Mutex<()> {
-    static GUARD: OnceLock<tokio::sync::Mutex<()>> = OnceLock::new();
-    GUARD.get_or_init(|| tokio::sync::Mutex::new(()))
-}
-
 struct TracingHarness {
     _server: MockServer,
     _codex_home: TempDir,
@@ -505,12 +501,8 @@ where
 }
 
 #[tokio::test(flavor = "current_thread")]
-#[expect(
-    clippy::await_holding_invalid_type,
-    reason = "test serializes access to global tracing state for its full duration"
-)]
+#[serial(app_server_tracing)]
 async fn thread_start_jsonrpc_span_exports_server_span_and_parents_children() -> Result<()> {
-    let _guard = tracing_test_guard().lock().await;
     let mut harness = TracingHarness::new().await?;
 
     let RemoteTrace {
@@ -588,12 +580,8 @@ async fn thread_start_jsonrpc_span_exports_server_span_and_parents_children() ->
 }
 
 #[tokio::test(flavor = "current_thread")]
-#[expect(
-    clippy::await_holding_invalid_type,
-    reason = "test serializes access to global tracing state for its full duration"
-)]
+#[serial(app_server_tracing)]
 async fn turn_start_jsonrpc_span_parents_core_turn_spans() -> Result<()> {
-    let _guard = tracing_test_guard().lock().await;
     let mut harness = TracingHarness::new().await?;
     let thread_start_response = harness.start_thread(/*request_id*/ 2, /*trace*/ None).await;
     let thread_id = thread_start_response.thread.id.clone();