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/tests/stream_no_completed.rs

@@ -4,6 +4,7 @@
 use std::time::Duration;
 
 use codex_core::Codex;
+use codex_core::CodexSpawnOk;
 use codex_core::ModelProviderInfo;
 use codex_core::exec::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
 use codex_core::protocol::EventMsg;
@@ -94,7 +95,7 @@ async fn retries_on_early_close() {
     let codex_home = TempDir::new().unwrap();
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider = model_provider;
-    let (codex, _init_id, _session_id) = Codex::spawn(config, ctrl_c).await.unwrap();
+    let CodexSpawnOk { codex, .. } = Codex::spawn(config, ctrl_c).await.unwrap();
 
     codex
         .submit(Op::UserInput {