chore: update Codex::spawn() to return a struct instead of a tuple (#1677)

Also update `init_codex()` to return a `struct` instead of a tuple, as well.

codex-rs/core/src/lib.rs

@@ -11,6 +11,7 @@ mod client;
 mod client_common;
 pub mod codex;
 pub use codex::Codex;
+pub use codex::CodexSpawnOk;
 pub mod codex_wrapper;
 pub mod config;
 pub mod config_profile;