Generate Python SDK types from pinned runtime

Make the SDK artifact generator fetch schema from the pinned runtime package, regenerate the checked-in Python types from that schema, and assert generated artifacts stay up to date.

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

sdk/python/tests/test_public_api_runtime_behavior.py

@@ -93,6 +93,7 @@ def _item_completed_notification(
         method="item/completed",
         payload=ItemCompletedNotification.model_validate(
             {
+                "completedAtMs": 1,
                 "item": item,
                 "threadId": thread_id,
                 "turnId": turn_id,