[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/tests/test_app_server_turn_controls.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from app_server_harness import AppServerHarness
 from app_server_helpers import agent_message_texts, streaming_response
 
-from openai_codex import Codex, TextInput
+from openai_codex import Codex
 from openai_codex.generated.v2_all import TurnStatus
 
 
@@ -21,9 +21,9 @@ def test_turn_steer_adds_follow_up_input(tmp_path) -> None:
 
         with Codex(config=harness.app_server_config()) as codex:
             thread = codex.thread_start()
-            turn = thread.turn(TextInput("Start a steerable turn."))
+            turn = thread.turn("Start a steerable turn.")
             harness.responses.wait_for_requests(1)
-            steer = turn.steer(TextInput("Use this steering input."))
+            steer = turn.steer("Use this steering input.")
             events = list(turn.stream())
             requests = harness.responses.wait_for_requests(2)
 
@@ -61,7 +61,7 @@ def test_turn_interrupt_stops_active_turn_and_follow_up_runs(tmp_path) -> None:
 
         with Codex(config=harness.app_server_config()) as codex:
             thread = codex.thread_start()
-            interrupted_turn = thread.turn(TextInput("Start a long turn."))
+            interrupted_turn = thread.turn("Start a long turn.")
             harness.responses.wait_for_requests(1)
             interrupt_response = interrupted_turn.interrupt()
             completed = interrupted_turn.run()