[codex] Return TurnResult from Python turn handles

[aibrahim-oai]

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.