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/src/models.rs

@@ -163,6 +163,7 @@ impl std::ops::Deref for FunctionCallOutputPayload {
 
 #[cfg(test)]
 mod tests {
+    #![allow(clippy::unwrap_used)]
     use super::*;
 
     #[test]