[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/10_error_handling_and_retry/async.py

@@ -18,7 +18,6 @@ from openai_codex import (
     AsyncCodex,
     JsonRpcError,
     ServerBusyError,
-    TextInput,
     is_retryable_error,
 )
 from openai_codex.types import TurnStatus
@@ -82,7 +81,7 @@ async def main() -> None:
 
 def _run_turn(thread, prompt: str):
     async def _inner():
-        turn = await thread.turn(TextInput(prompt))
+        turn = await thread.turn(prompt)
         return await turn.run()
 
     return _inner

sdk/python/examples/10_error_handling_and_retry/sync.py

@@ -13,7 +13,6 @@ from openai_codex import (
     Codex,
     JsonRpcError,
     ServerBusyError,
-    TextInput,
     retry_on_overload,
 )
 from openai_codex.types import TurnStatus
@@ -23,7 +22,7 @@ with Codex(config=runtime_config()) as codex:
 
     try:
         result = retry_on_overload(
-            lambda: thread.turn(TextInput("Summarize retry best practices in 3 bullets.")).run(),
+            lambda: thread.turn("Summarize retry best practices in 3 bullets.").run(),
             max_attempts=3,
             initial_delay_s=0.25,
             max_delay_s=2.0,