[ray.data.llm] Update release test: enforce pp_backend

[lk-chen]

Why are these changes needed?

PP backend argument is defaulted to "mp", but as of vLLM 0.8.5, vLLM V1 does not accept "mp" and requires "ray", otherwise it will fall back to V0.

There's recent vllm-project/vllm#14219 to come up in next release, to support "mp" in V1.

Our release test should catch "ray" backend only, hence this PR.

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