Added allow-expect-in-tests / allow-unwrap-in-tests (#2328)

This PR:
* Added the clippy.toml to configure allowable expect / unwrap usage in
tests
* Removed as many expect/allow lines as possible from tests
* moved a bunch of allows to expects where possible

Note: in integration tests, non `#[test]` helper functions are not
covered by this so we had to leave a few lingering `expect(expect_used`
checks around

codex-rs/core/tests/live_cli.rs

@@ -17,7 +17,7 @@ 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)]
+    #![expect(clippy::unwrap_used)]
     use std::io::Read;
     use std::io::Write;
     use std::thread;
@@ -113,7 +113,6 @@ 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;