[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/scripts/update_sdk_artifacts.py

@@ -1040,12 +1040,12 @@ def _render_thread_block(
     lines = [
         "    def turn(",
         "        self,",
-        "        input: Input,",
+        "        input: RunInput,",
         "        *,",
         *_approval_mode_override_signature_lines(),
         *_kw_signature_lines(turn_fields),
         "    ) -> TurnHandle:",
-        "        wire_input = _to_wire_input(input)",
+        "        wire_input = _to_wire_input(_normalize_run_input(input))",
         _approval_mode_assignment_line("_approval_mode_override_settings"),
         "        params = TurnStartParams(",
         "            thread_id=self.id,",
@@ -1065,13 +1065,13 @@ def _render_async_thread_block(
     lines = [
         "    async def turn(",
         "        self,",
-        "        input: Input,",
+        "        input: RunInput,",
         "        *,",
         *_approval_mode_override_signature_lines(),
         *_kw_signature_lines(turn_fields),
         "    ) -> AsyncTurnHandle:",
         "        await self._codex._ensure_initialized()",
-        "        wire_input = _to_wire_input(input)",
+        "        wire_input = _to_wire_input(_normalize_run_input(input))",
         _approval_mode_assignment_line("_approval_mode_override_settings"),
         "        params = TurnStartParams(",
         "            thread_id=self.id,",