fix(codex): align OAuth defaults and setup docs (#4910)

* fix(codex): align OAuth defaults and setup docs

* docs(codex): clarify provider settings key

* docs(codex): simplify OAuth setup guidance

tests/providers/test_openai_codex_provider.py

@@ -18,6 +18,7 @@ from nanobot.providers.openai_codex_provider import (
     _request_codex,
     _should_retry_status,
 )
+from nanobot.providers.registry import find_by_name
 
 
 def _mock_codex_token(monkeypatch: pytest.MonkeyPatch) -> None:
@@ -30,6 +31,14 @@ def _mock_codex_token(monkeypatch: pytest.MonkeyPatch) -> None:
     )
 
 
+def test_codex_default_model_matches_curated_flagship() -> None:
+    spec = find_by_name("openai_codex")
+
+    assert spec is not None
+    assert spec.builtin_models
+    assert OpenAICodexProvider().get_default_model() == spec.builtin_models[0].id
+
+
 class _WarningCaptureLogger:
     def __init__(self) -> None:
         self.calls: list[tuple[str, tuple[Any, ...]]] = []