[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/05_existing_thread/sync.py

@@ -5,12 +5,7 @@ _EXAMPLES_ROOT = Path(__file__).resolve().parents[1]
 if str(_EXAMPLES_ROOT) not in sys.path:
     sys.path.insert(0, str(_EXAMPLES_ROOT))
 
-from _bootstrap import (
-    assistant_text_from_turn,
-    ensure_local_sdk_src,
-    find_turn_by_id,
-    runtime_config,
-)
+from _bootstrap import ensure_local_sdk_src, runtime_config
 
 ensure_local_sdk_src()
 
@@ -25,6 +20,4 @@ with Codex(config=runtime_config()) as codex:
     # Resume the existing thread by ID.
     resumed = codex.thread_resume(original.id)
     second = resumed.turn(TextInput("Continue with one more fact.")).run()
-    persisted = resumed.read(include_turns=True)
-    persisted_turn = find_turn_by_id(persisted.thread.turns, second.id)
-    print(assistant_text_from_turn(persisted_turn))
+    print(second.final_response)