[CI/Build] mypy: resolve some error from vllm/entrypoints

[iamrajhans]

This PR is a chunk of mypy checking progress

part of #3680

[github-actions]

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[njhill]

Thanks @iamrajhans, but actually this will be resolved by #9473. The typing error here was caused by a change to the type of RequestOutput.prompt which that PR reverts.