fix(types): narrow await_args for mypy in x-cashu key-config test

uv run mypy . failed with union-attr on store_mock.await_args.kwargs
(AsyncMock.await_args is typed _Call | None, and assert_awaited_once()
does not narrow it). Add an explicit None check before accessing kwargs.

tests/unit/test_tinfoil_integration.py

@@ -1028,6 +1028,7 @@ async def test_x_cashu_key_config_422_refunds_and_sets_x_cashu_header() -> None:
     )
     # The redemption itself was recorded.
     store_mock.assert_awaited_once()
+    assert store_mock.await_args is not None
     assert store_mock.await_args.kwargs.get("typ") == "in"
     # Passthrough shape with the refund attached.
     assert response.status_code == 422