[Feature] Add nightly test for diffusion LLMs(like LLaDA-2-flash 100B)

[Monstertail]

Checklist

• If this is not a feature request but a general question, please start a discussion at https://github.com/sgl-project/sglang/discussions. Otherwise, it will be closed.
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Motivation

We are supporting the normal CI tests for small diffusion LLMs.
In the discussion here, we need a nightly test for large scale dLLMs like LLaDA-2-flash 100B in the future: #14723 (comment) .

@btw616 @ClawSeven

Related resources

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