[app-server] model/list API (#5382)

Adds a `model/list` paginated API that returns the list of models
supported by Codex.

codex-rs/app-server/tests/suite/mod.rs

@@ -7,6 +7,7 @@ mod fuzzy_file_search;
 mod interrupt;
 mod list_resume;
 mod login;
+mod model_list;
 mod rate_limits;
 mod send_message;
 mod set_default_model;