[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/examples/_bootstrap.py

@@ -6,7 +6,7 @@ import sys
 import tempfile
 import zlib
 from pathlib import Path
-from typing import Iterable, Iterator
+from typing import Any, Iterator
 
 _SDK_PYTHON_DIR = Path(__file__).resolve().parents[1]
 _SDK_PYTHON_STR = str(_SDK_PYTHON_DIR)
@@ -103,53 +103,5 @@ def temporary_sample_image_path() -> Iterator[Path]:
         yield image_path
 
 
-def server_label(metadata: object) -> str:
-    server = getattr(metadata, "serverInfo", None)
-    server_name = ((getattr(server, "name", None) or "") if server is not None else "").strip()
-    server_version = (
-        (getattr(server, "version", None) or "") if server is not None else ""
-    ).strip()
-    if server_name and server_version:
-        return f"{server_name} {server_version}"
-
-    user_agent = (
-        (getattr(metadata, "userAgent", None) or "") if metadata is not None else ""
-    ).strip()
-    return user_agent or "unknown"
-
-
-def find_turn_by_id(turns: Iterable[object] | None, turn_id: str) -> object | None:
-    for turn in turns or []:
-        if getattr(turn, "id", None) == turn_id:
-            return turn
-    return None
-
-
-def assistant_text_from_turn(turn: object | None) -> str:
-    if turn is None:
-        return ""
-
-    chunks: list[str] = []
-    for item in getattr(turn, "items", []) or []:
-        raw_item = item.model_dump(mode="json") if hasattr(item, "model_dump") else item
-        if not isinstance(raw_item, dict):
-            continue
-
-        item_type = raw_item.get("type")
-        if item_type == "agentMessage":
-            text = raw_item.get("text")
-            if isinstance(text, str) and text:
-                chunks.append(text)
-            continue
-
-        if item_type != "message" or raw_item.get("role") != "assistant":
-            continue
-
-        for content in raw_item.get("content") or []:
-            if not isinstance(content, dict) or content.get("type") != "output_text":
-                continue
-            text = content.get("text")
-            if isinstance(text, str) and text:
-                chunks.append(text)
-
-    return "".join(chunks)
+def server_label(metadata: Any) -> str:
+    return f"{metadata.serverInfo.name} {metadata.serverInfo.version}"