[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/docs/getting-started.md

@@ -2,7 +2,7 @@
 
 This is the fastest path from install to a multi-turn thread using the public SDK surface.
 
-The SDK is experimental. Treat the API, bundled runtime strategy, and packaging details as unstable until the first public release.
+The SDK is experimental, so the public API and runtime requirements may keep evolving before the first public release.
 
 ## 1) Install