[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_contract_generation.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import importlib.metadata
 import os
 import subprocess
 import sys
@@ -14,6 +15,7 @@ GENERATED_TARGETS = [
 
 
 def _snapshot_target(root: Path, rel_path: Path) -> dict[str, bytes] | bytes | None:
+    """Capture one generated artifact so regeneration drift is easy to compare."""
     target = root / rel_path
     if not target.exists():
         return None
@@ -28,16 +30,22 @@ def _snapshot_target(root: Path, rel_path: Path) -> dict[str, bytes] | bytes | N
 
 
 def _snapshot_targets(root: Path) -> dict[str, dict[str, bytes] | bytes | None]:
+    """Capture all checked-in generated artifacts before and after regeneration."""
     return {
-        str(rel_path): _snapshot_target(root, rel_path) for rel_path in GENERATED_TARGETS
+        str(rel_path): _snapshot_target(root, rel_path)
+        for rel_path in GENERATED_TARGETS
     }
 
 
 def test_generated_files_are_up_to_date():
+    """Regenerating from the pinned runtime package should leave artifacts unchanged."""
     before = _snapshot_targets(ROOT)
 
-    # Regenerate contract artifacts via single maintenance entrypoint.
+    # Regenerate contract artifacts via the pinned runtime package, not a local
+    # app-server binary from the checkout or CI environment.
+    assert importlib.metadata.version("openai-codex-cli-bin") == "0.131.0a4"
     env = os.environ.copy()
+    env.pop("CODEX_EXEC_PATH", None)
     python_bin = str(Path(sys.executable).parent)
     env["PATH"] = f"{python_bin}{os.pathsep}{env.get('PATH', '')}"