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/live_agent.rs

@@ -20,6 +20,7 @@
 use std::time::Duration;
 
 use codex_core::Codex;
+use codex_core::CodexSpawnOk;
 use codex_core::error::CodexErr;
 use codex_core::protocol::AgentMessageEvent;
 use codex_core::protocol::ErrorEvent;
@@ -48,7 +49,7 @@ async fn spawn_codex() -> Result<Codex, CodexErr> {
     let mut config = load_default_config_for_test(&codex_home);
     config.model_provider.request_max_retries = Some(2);
     config.model_provider.stream_max_retries = Some(2);
-    let (agent, _init_id, _session_id) =
+    let CodexSpawnOk { codex: agent, .. } =
         Codex::spawn(config, std::sync::Arc::new(Notify::new())).await?;
 
     Ok(agent)