[codex] Accept string input for Python turns (#23162)

## 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"

sdk/python/examples/05_existing_thread/async.py

@@ -11,7 +11,7 @@ ensure_local_sdk_src()
 
 import asyncio
 
-from openai_codex import AsyncCodex, TextInput
+from openai_codex import AsyncCodex
 
 
 async def main() -> None:
@@ -20,12 +20,12 @@ async def main() -> None:
             model="gpt-5.4", config={"model_reasoning_effort": "high"}
         )
 
-        first_turn = await original.turn(TextInput("Tell me one fact about Saturn."))
+        first_turn = await original.turn("Tell me one fact about Saturn.")
         _ = await first_turn.run()
         print("Created thread:", original.id)
 
         resumed = await codex.thread_resume(original.id)
-        second_turn = await resumed.turn(TextInput("Continue with one more fact."))
+        second_turn = await resumed.turn("Continue with one more fact.")
         second = await second_turn.run()
         print(second.final_response)