fix: enable clippy on tests (#870)

https://github.com/openai/codex/pull/855 added the clippy warning to
disallow `unwrap()`, but apparently we were not verifying that tests
were "clippy clean" in CI, so I ended up with a lot of local errors in
VS Code.

This turns on the check in CI and fixes the offenders.

codex-rs/core/tests/previous_response_id.rs

@@ -48,6 +48,8 @@ data: {{\"type\":\"response.completed\",\"response\":{{\"id\":\"{}\",\"output\":
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn keeps_previous_response_id_between_tasks() {
+    #![allow(clippy::unwrap_used)]
+
     // Mock server
     let server = MockServer::start().await;