Adapt the latest sglang integration test Engine kwargs.

[changwangss]

Description

Root cause: sglang 0.5.14 removed legacy Engine/ServerArgs kwargs (disable_piecewise_cuda_graph, cuda_graph_bs) in favor of the new CUDA graph config fields, so the old test arguments became invalid.

I improved ut as following.

• Fix sglang integration test by adapting Engine kwargs to ServerArgs changes in newer sglang.
• Use legacy args when available, otherwise fall back to new args (disable_cuda_graph / cuda_graph_bs_decode).
• Keeps backward compatibility and restores test_ar_format_sglang on sglang 0.5.14.

Type of Change

Bug fix

#1961

Fixes or relates to #

Checklist Before Submitting

• My code has been tested locally.
• Documentation has been updated as needed.
• New or updated tests are included where applicable.
• The CUDA CI has passed. You can trigger it by commenting /azp run Unit-Test-CUDA-AutoRound.