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/examples/01_quickstart_constructor/async.py

@@ -6,9 +6,7 @@ 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,
     server_label,
 )
@@ -17,7 +15,7 @@ ensure_local_sdk_src()
 
 import asyncio
 
-from codex_app_server import AsyncCodex, TextInput
+from codex_app_server import AsyncCodex
 
 
 async def main() -> None:
@@ -25,13 +23,9 @@ async def main() -> None:
         print("Server:", server_label(codex.metadata))
 
         thread = await codex.thread_start(model="gpt-5.4", config={"model_reasoning_effort": "high"})
-        turn = await thread.turn(TextInput("Say hello in one sentence."))
-        result = await turn.run()
-        persisted = await thread.read(include_turns=True)
-        persisted_turn = find_turn_by_id(persisted.thread.turns, result.id)
-
-        print("Status:", result.status)
-        print("Text:", assistant_text_from_turn(persisted_turn))
+        result = await thread.run("Say hello in one sentence.")
+        print("Items:", len(result.items))
+        print("Text:", result.final_response)
 
 
 if __name__ == "__main__":