[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/06_thread_lifecycle_and_controls/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:
@@ -19,10 +19,8 @@ async def main() -> None:
         thread = await codex.thread_start(
             model="gpt-5.4", config={"model_reasoning_effort": "high"}
         )
-        first = await (
-            await thread.turn(TextInput("One sentence about structured planning."))
-        ).run()
-        second = await (await thread.turn(TextInput("Now restate it for a junior engineer."))).run()
+        first = await (await thread.turn("One sentence about structured planning.")).run()
+        second = await (await thread.turn("Now restate it for a junior engineer.")).run()
 
         reopened = await codex.thread_resume(thread.id)
         listing_active = await codex.thread_list(limit=20, archived=False)
@@ -38,13 +36,11 @@ async def main() -> None:
             model="gpt-5.4",
             config={"model_reasoning_effort": "high"},
         )
-        resumed_result = await (
-            await resumed.turn(TextInput("Continue in one short sentence."))
-        ).run()
+        resumed_result = await (await resumed.turn("Continue in one short sentence.")).run()
 
         forked = await codex.thread_fork(unarchived.id, model="gpt-5.4")
         forked_result = await (
-            await forked.turn(TextInput("Take a different angle in one short sentence."))
+            await forked.turn("Take a different angle in one short sentence.")
         ).run()
 
         compact_result = await unarchived.compact()

sdk/python/examples/06_thread_lifecycle_and_controls/sync.py

@@ -9,12 +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"})
-    first = thread.turn(TextInput("One sentence about structured planning.")).run()
-    second = thread.turn(TextInput("Now restate it for a junior engineer.")).run()
+    first = thread.turn("One sentence about structured planning.").run()
+    second = thread.turn("Now restate it for a junior engineer.").run()
 
     reopened = codex.thread_resume(thread.id)
     listing_active = codex.thread_list(limit=20, archived=False)
@@ -30,10 +30,10 @@ with Codex(config=runtime_config()) as codex:
         model="gpt-5.4",
         config={"model_reasoning_effort": "high"},
     )
-    resumed_result = resumed.turn(TextInput("Continue in one short sentence.")).run()
+    resumed_result = resumed.turn("Continue in one short sentence.").run()
 
     forked = codex.thread_fork(unarchived.id, model="gpt-5.4")
-    forked_result = forked.turn(TextInput("Take a different angle in one short sentence.")).run()
+    forked_result = forked.turn("Take a different angle in one short sentence.").run()
 
     compact_result = unarchived.compact()