[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/06_thread_lifecycle_and_controls/sync.py

@@ -25,40 +25,24 @@ with Codex(config=runtime_config()) as codex:
     listing_archived = codex.thread_list(limit=20, archived=True)
     unarchived = codex.thread_unarchive(reopened.id)
 
-    resumed_info = "n/a"
-    try:
-        resumed = codex.thread_resume(
-            unarchived.id,
-            model="gpt-5.4",
-            config={"model_reasoning_effort": "high"},
-        )
-        resumed_result = resumed.turn(TextInput("Continue in one short sentence.")).run()
-        resumed_info = f"{resumed_result.id} {resumed_result.status}"
-    except Exception as exc:
-        resumed_info = f"skipped({type(exc).__name__})"
+    resumed = codex.thread_resume(
+        unarchived.id,
+        model="gpt-5.4",
+        config={"model_reasoning_effort": "high"},
+    )
+    resumed_result = resumed.turn(TextInput("Continue in one short sentence.")).run()
 
-    forked_info = "n/a"
-    try:
-        forked = codex.thread_fork(unarchived.id, model="gpt-5.4")
-        forked_result = forked.turn(
-            TextInput("Take a different angle in one short sentence.")
-        ).run()
-        forked_info = f"{forked_result.id} {forked_result.status}"
-    except Exception as exc:
-        forked_info = f"skipped({type(exc).__name__})"
+    forked = codex.thread_fork(unarchived.id, model="gpt-5.4")
+    forked_result = forked.turn(TextInput("Take a different angle in one short sentence.")).run()
 
-    compact_info = "sent"
-    try:
-        _ = unarchived.compact()
-    except Exception as exc:
-        compact_info = f"skipped({type(exc).__name__})"
+    compact_result = unarchived.compact()
 
     print("Lifecycle OK:", thread.id)
     print("first:", first.id, first.status)
     print("second:", second.id, second.status)
-    print("read.turns:", len(reading.thread.turns or []))
+    print("read.turns:", len(reading.thread.turns))
     print("list.active:", len(listing_active.data))
     print("list.archived:", len(listing_archived.data))
-    print("resumed:", resumed_info)
-    print("forked:", forked_info)
-    print("compact:", compact_info)
+    print("resumed:", resumed_result.id, resumed_result.status)
+    print("forked:", forked_result.id, forked_result.status)
+    print("compact:", compact_result.model_dump(mode="json", by_alias=True))