[codex] Refine Python SDK user-facing docs

[aibrahim-oai]

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.