test(integration): add list_value helper

Prerequisite for the backport of #46099 to stable/1.103.x: its integration
tests import list_value, which reached main in #43981.
Hand-ported from 43f07d0bd8 (main).

tests/integration/_support/client.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import os
 import time
 import uuid
-from collections.abc import Callable, Iterator, Mapping
+from collections.abc import Callable, Iterator, Mapping, Sequence
 from contextlib import ExitStack, contextmanager
 from dataclasses import dataclass
 from hashlib import sha256
@@ -22,6 +22,11 @@ def object_value(value: JsonValue) -> dict[str, JsonValue]:
     return JSON_OBJECT.validate_python(value)
 
 
+def list_value(value: JsonValue) -> Sequence[JsonValue]:
+    assert isinstance(value, list), f"Expected a list, received {type(value).__name__}"
+    return value
+
+
 def string_value(value: JsonValue) -> str:
     assert isinstance(value, str), f"Expected a string, received {type(value).__name__}"
     return value