[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/sync.py

@@ -9,15 +9,15 @@ from _bootstrap import ensure_local_sdk_src, runtime_config
 
 ensure_local_sdk_src()
 
-from openai_codex import Codex, TextInput
+from openai_codex import Codex
 
 with Codex(config=runtime_config()) as codex:
     # Create an initial thread and turn so we have a real thread to resume.
     original = codex.thread_start(model="gpt-5.4", config={"model_reasoning_effort": "high"})
-    first = original.turn(TextInput("Tell me one fact about Saturn.")).run()
+    first = original.turn("Tell me one fact about Saturn.").run()
     print("Created thread:", original.id)
 
     # Resume the existing thread by ID.
     resumed = codex.thread_resume(original.id)
-    second = resumed.turn(TextInput("Continue with one more fact.")).run()
+    second = resumed.turn("Continue with one more fact.").run()
     print(second.final_response)