[codex] Accept string input for Python turns

[aibrahim-oai]

Summary

• Allow thread.turn and turn.steer, including async variants, to accept RunInput so plain strings work alongside typed input objects.
• Export RunInput and update the SDK artifact generator so regenerated turn methods keep the same signature and normalization.
• Update docs, examples, notebook cells, and tests to use string shorthand for text-only turns while keeping typed inputs for multimodal input.

Validation

• uv run --extra dev ruff format .
• uv run --extra dev ruff check --output-format=github .
• python3 -m py_compile sdk/python/src/openai_codex/init.py sdk/python/src/openai_codex/api.py sdk/python/src/openai_codex/_inputs.py sdk/python/scripts/update_sdk_artifacts.py sdk/python/tests/test_public_api_signatures.py sdk/python/tests/test_app_server_streaming.py sdk/python/tests/test_app_server_turn_controls.py sdk/python/tests/test_real_app_server_integration.py
• python3 -c "import json; json.load(open('sdk/python/notebooks/sdk_walkthrough.ipynb'))"
• sdk/python/.venv/bin/python -c "import inspect, openai_codex; from openai_codex import Thread, AsyncThread, TurnHandle, AsyncTurnHandle, RunInput; funcs=[Thread.run, Thread.turn, AsyncThread.run, AsyncThread.turn, TurnHandle.steer, AsyncTurnHandle.steer]; assert all(inspect.signature(fn).parameters['input'].annotation == 'RunInput' for fn in funcs); assert RunInput is openai_codex.RunInput"