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/live_cli.rs

@@ -15,6 +15,7 @@ fn require_api_key() -> String {
 
 /// Helper that spawns the binary inside a TempDir with minimal flags. Returns (Assert, TempDir).
 fn run_live(prompt: &str) -> (assert_cmd::assert::Assert, TempDir) {
+    #![allow(clippy::unwrap_used)]
     use std::io::Read;
     use std::io::Write;
     use std::thread;
@@ -110,6 +111,7 @@ fn run_live(prompt: &str) -> (assert_cmd::assert::Assert, TempDir) {
 #[ignore]
 #[test]
 fn live_create_file_hello_txt() {
+    #![allow(clippy::unwrap_used)]
     if std::env::var("OPENAI_API_KEY").is_err() {
         eprintln!("skipping live_create_file_hello_txt – OPENAI_API_KEY not set");
         return;