Add Python SDK thread.run convenience methods (#15088)

## TL;DR
Add `thread.run(...)` / `async thread.run(...)` convenience methods to
the Python SDK for the common case.

- add `RunInput = Input | str` and `RunResult` with `final_response`,
collected `items`, and optional `usage`
- keep `thread.turn(...)` strict and lower-level for streaming,
steering, interrupting, and raw generated `Turn` access
- update Python SDK docs, quickstart examples, and tests for the sync
and async convenience flows

## Validation
- `python3 -m pytest sdk/python/tests/test_public_api_signatures.py
sdk/python/tests/test_public_api_runtime_behavior.py`
- `python3 -m pytest
sdk/python/tests/test_real_app_server_integration.py -k
'thread_run_convenience or async_thread_run_convenience'` (skipped in
this environment)

---------

Co-authored-by: Codex <noreply@openai.com>

sdk/python/src/codex_app_server/__init__.py

@@ -47,6 +47,7 @@ from .api import (
     InputItem,
     LocalImageInput,
     MentionInput,
+    RunResult,
     SkillInput,
     TextInput,
     Thread,
@@ -68,6 +69,7 @@ __all__ = [
     "TurnHandle",
     "AsyncTurnHandle",
     "InitializeResponse",
+    "RunResult",
     "Input",
     "InputItem",
     "TextInput",