Add models endpoint (#7603)

- Use the codex-api crate to introduce models endpoint. 
- Add `models` to codex core tests helpers
- Add `ModelsInfo` for the endpoint return type

codex-rs/codex-api/src/endpoint/mod.rs

@@ -1,4 +1,5 @@
 pub mod chat;
 pub mod compact;
+pub mod models;
 pub mod responses;
 mod streaming;