[6/6] Default Python SDK approval policy to never

[aibrahim-oai]

Summary

• Force SDK thread and turn starts to serialize AskForApproval.never until SDK approval request handling exists.
• Add generated AskForApproval aliases such as AskForApproval.never and keep the generator preserving them.
• Update examples/docs and add sync + async coverage for the forced approval policy.

Stack

• [1/6] [1/6] Pin Python SDK runtime dependency #21891 Pin Python SDK runtime dependency
• [2/6] [2/6] Generate Python SDK types from pinned runtime #21893 Generate Python SDK types from pinned runtime
• [3/6] [3/6] Run Python SDK tests in CI #21895 Run Python SDK tests in CI
• [4/6] [4/6] Define Python SDK public API surface #21896 Define Python SDK public API surface
• [5/6] [5/6] Rename Python SDK package to openai-codex #21905 Rename Python SDK package to openai-codex
• [6/6] [6/6] Default Python SDK approval policy to never #21910 Default Python SDK approval policy to never

This PR is the sixth change in the stack, on top of #21905.

Verification

• PYTHONPATH=sdk/python/src python3 -c ... for AskForApproval.never serialization
• python3 -m py_compile ... on changed Python files
• git diff --check
• Local pytest not run per repo guidance; CI should cover it.