chore: change CodexAuth::from_api_key() to take &str instead of String (#1970)

Good practice and simplifies some of the call sites.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/1970).
* #1971
* __->__ #1970
* #1966
* #1965
* #1962

codex-rs/core/tests/stream_no_completed.rs

@@ -99,7 +99,7 @@ async fn retries_on_early_close() {
     config.model_provider = model_provider;
     let CodexSpawnOk { codex, .. } = Codex::spawn(
         config,
-        Some(CodexAuth::from_api_key("Test API Key".to_string())),
+        Some(CodexAuth::from_api_key("Test API Key")),
         ctrl_c,
     )
     .await