[codex] Return TurnResult from Python turn handles (#23151)

## Why

`TurnHandle.run()` returned the raw app-server `Turn`, whose live
start/completed payloads do not include loaded `items`, so users saw
empty `items` after starting a turn. That made the handle-based path
behave differently from `Thread.run(...)`, and pushed examples toward
persisted-thread reads plus helper extraction.

This PR makes the run APIs standalone: starting a turn and running it
returns collected turn data directly, or fails visibly when required
stream events are missing.

## What Changed

- Replaces the public `RunResult` export with `TurnResult`.
- Adds turn metadata to `TurnResult`: `id`, `status`, `error`,
`started_at`, `completed_at`, and `duration_ms`, alongside
`final_response`, `items`, and `usage`.
- Changes `TurnHandle.run()` and `AsyncTurnHandle.run()` to consume
stream events with the same collector used by `Thread.run(...)`.
- Exports `TurnError` from `openai_codex.types` for the new result
shape.
- Updates tests, examples, docs, and the walkthrough notebook to use
`result.final_response` and `result.items` directly.
- Removes persisted-thread helper paths and placeholder/skipped control
flows from the public examples and notebook.

## Verification

- `python3 -m py_compile ...` over changed SDK, example, and test Python
files.
- `python3 -c "import json;
json.load(open('sdk/python/notebooks/sdk_walkthrough.ipynb'))"`
- `git diff --check`
- `PYTHONPATH=sdk/python/src python3 -c ...` import/signature smoke for
`TurnResult`, `TurnHandle.run`, and `AsyncTurnHandle.run`.

sdk/python/tests/test_app_server_streaming.py

@@ -5,6 +5,7 @@ import asyncio
 from app_server_harness import AppServerHarness
 from app_server_helpers import (
     agent_message_texts,
+    agent_message_texts_from_items,
     next_async_delta,
     next_sync_delta,
     streaming_response,
@@ -48,7 +49,7 @@ def test_sync_stream_routes_text_deltas_and_completion(tmp_path) -> None:
 
 
 def test_turn_run_returns_completed_turn(tmp_path) -> None:
-    """TurnHandle.run should wait for the app-server completion notification."""
+    """TurnHandle.run should collect output and completion metadata."""
     with AppServerHarness(tmp_path) as harness:
         harness.responses.enqueue_assistant_message("turn complete", response_id="turn-run-1")
 
@@ -60,11 +61,13 @@ def test_turn_run_returns_completed_turn(tmp_path) -> None:
     assert {
         "turn_id": completed.id,
         "status": completed.status,
-        "items": completed.items,
+        "agent_messages": agent_message_texts_from_items(completed.items),
+        "final_response": completed.final_response,
     } == {
         "turn_id": turn.id,
         "status": TurnStatus.completed,
-        "items": [],
+        "agent_messages": ["turn complete"],
+        "final_response": "turn complete",
     }