[2/8] Generate Python SDK types from pinned runtime (#21893)

## Why

Once the SDK declares its runtime package, generated Python artifacts
should come from that pinned runtime rather than whatever app-server
schema happens to be in the current checkout. That keeps the generated
API and model surface aligned with the runtime users install.

## What

- Teach `scripts/update_sdk_artifacts.py generate-types` to invoke the
pinned runtime package for schema generation.
- Regenerate `v2_all.py`, `notification_registry.py`, and generated
public wrapper methods from that schema.
- Add freshness coverage so regenerating from the pinned runtime must
leave checked-in artifacts unchanged.

## Stack

1. #21891 `[1/8]` Pin Python SDK runtime dependency
2. This PR `[2/8]` Generate Python SDK types from pinned runtime
3. #21895 `[3/8]` Run Python SDK tests in CI
4. #21896 `[4/8]` Define Python SDK public API surface
5. #21905 `[5/8]` Rename Python SDK package to `openai-codex`
6. #21910 `[6/8]` Add high-level Python SDK approval mode
7. #22014 `[7/8]` Add Python SDK app-server integration harness
8. #22021 `[8/8]` Add Python SDK Ruff formatting

## Verification

- Added `test_generated_files_are_up_to_date` for pinned-runtime
generation drift.
- Added generator-structure tests for schema annotation and notification
metadata generation.

---------

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

sdk/python/tests/test_public_api_runtime_behavior.py

@@ -82,6 +82,7 @@ def _item_completed_notification(
     text: str = "final text",
     phase: MessagePhase | None = None,
 ) -> Notification:
+    """Build a realistic completed-item notification accepted by generated models."""
     item: dict[str, object] = {
         "id": "item-1",
         "text": text,
@@ -93,6 +94,8 @@ def _item_completed_notification(
         method="item/completed",
         payload=ItemCompletedNotification.model_validate(
             {
+                # The pinned runtime schema requires completion timestamps.
+                "completedAtMs": 1,
                 "item": item,
                 "threadId": thread_id,
                 "turnId": turn_id,