test(django-cf): add basic async handlers test for d1 and r2

[ryanking13]

On top of #198.

Add basic test coverage of using d1, r2 backends with async handlers.

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

This PR adds async (ASGI) test coverage for the D1 and R2 backends, with a shared _asgi_client.py helper, five D1 ORM tests exercising create/read/count/exists/update/iterate, and one R2 save-and-read test.

Issues found (1):

1. test_asgi_d1_orm_async_iteration_returns_all_rows assertion masks order_by bugs (test_asgi_d1.py:156): The view applies .order_by("value") but the assertion wraps the result in sorted(), so it would pass even if ordering were broken. Posted as an inline suggestion.

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