[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_lifecycle.py

@@ -157,7 +157,7 @@ def test_async_lifecycle_methods_round_trip(tmp_path) -> None:
 
             async with AsyncCodex(config=harness.app_server_config()) as codex:
                 thread = await codex.thread_start()
-                run_result = await thread.run("materialize async thread")
+                turn_result = await thread.run("materialize async thread")
                 await thread.set_name("async lifecycle")
                 named = await thread.read()
                 resumed = await codex.thread_resume(thread.id)
@@ -166,14 +166,14 @@ def test_async_lifecycle_methods_round_trip(tmp_path) -> None:
                 unarchived = await codex.thread_unarchive(thread.id)
 
         assert {
-            "run_final_response": run_result.final_response,
+            "turn_final_response": turn_result.final_response,
             "named_thread": named.thread.name,
             "resumed_id": resumed.id,
             "forked_is_distinct": forked.id != thread.id,
             "archive_response": archive_response.model_dump(by_alias=True, mode="json"),
             "unarchived_id": unarchived.id,
         } == {
-            "run_final_response": "async materialized",
+            "turn_final_response": "async materialized",
             "named_thread": "async lifecycle",
             "resumed_id": thread.id,
             "forked_is_distinct": True,
@@ -253,19 +253,19 @@ def test_compact_rpc_hits_mock_responses(tmp_path) -> None:
 
         with Codex(config=harness.app_server_config()) as codex:
             thread = codex.thread_start()
-            run_result = thread.run("create history")
+            turn_result = thread.run("create history")
             compact_response = thread.compact()
             requests = harness.responses.wait_for_requests(2)
 
     assert {
-        "run_final_response": run_result.final_response,
+        "turn_final_response": turn_result.final_response,
         "compact_response": compact_response.model_dump(
             by_alias=True,
             mode="json",
         ),
         "request_kinds": [request_kind(request.path) for request in requests],
     } == {
-        "run_final_response": "history",
+        "turn_final_response": "history",
         "compact_response": {},
         "request_kinds": ["responses", "responses"],
     }