[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/14_turn_controls/sync.py

@@ -9,14 +9,12 @@ 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:
     thread = codex.thread_start(model="gpt-5.4", config={"model_reasoning_effort": "high"})
-    steer_turn = thread.turn(
-        TextInput("Count from 1 to 40 with commas, then one summary sentence.")
-    )
-    steer_result = steer_turn.steer(TextInput("Keep it brief and stop after 10 numbers."))
+    steer_turn = thread.turn("Count from 1 to 40 with commas, then one summary sentence.")
+    steer_result = steer_turn.steer("Keep it brief and stop after 10 numbers.")
 
     steer_event_count = 0
     steer_completed_status = None
@@ -33,9 +31,7 @@ with Codex(config=runtime_config()) as codex:
         raise RuntimeError("stream ended without turn/completed")
     steer_preview = "".join(steer_deltas).strip()
 
-    interrupt_turn = thread.turn(
-        TextInput("Count from 1 to 200 with commas, then one summary sentence.")
-    )
+    interrupt_turn = thread.turn("Count from 1 to 200 with commas, then one summary sentence.")
     interrupt_result = interrupt_turn.interrupt()
 
     interrupt_event_count = 0