Update cargo to 2024 edition (#842)

Some effects of this change:
- New formatting changes across many files. No functionality changes
should occur from that.
- Calls to `set_env` are considered unsafe, since this only happens in
tests we wrap them in `unsafe` blocks

codex-rs/core/src/models.rs

@@ -1,7 +1,7 @@
 use base64::Engine;
-use serde::ser::Serializer;
 use serde::Deserialize;
 use serde::Serialize;
+use serde::ser::Serializer;
 
 use crate::protocol::InputItem;