[codex] Refine Python SDK user-facing docs (#22941)

## Summary
- Remove maintainer and release-process wording from the Python SDK
README and docs.
- Rewrite SDK-facing comments/docstrings so they read as standalone
product documentation.
- Add a real app-server integration smoke that follows the public
quickstart-style `Codex() -> thread_start() -> run()` path.

## Integration coverage
- Add `test_real_quickstart_style_flow_smoke` in the real app-server
integration suite.

## Validation
- Local tests were not run per repo guidance. CI should validate this
branch once the PR is online.

sdk/python/src/openai_codex/api.py

@@ -113,7 +113,7 @@ def _approval_mode_override_settings(
 
 
 class Codex:
-    """Minimal typed SDK surface for app-server v2."""
+    """Typed Python client for app-server v2 workflows."""
 
     def __init__(self, config: AppServerConfig | None = None) -> None:
         self._client = AppServerClient(config=config)