[Bug Fix] Re-add the DCP/PCP compatibility check for CUDA platform

[Siritao]

Background:

• PR [PCP&DCP] move CUDAGraph check for PCP&DCP to the check func of platforms #29952 moved DCP/PCP + full CUDA graphs compatibility check to platform layer
• PR [Perf] Enable cuda graph for deepepHT, 5.3% throughput improvement, 4.4% TTFT improvement #29558 inadvertently removed this check during conflict resolution (see commit diff)
• ROCm platform still has the check, but CUDA is missing it
• Impact: Users with DCP/PCP enabled crash during full CUDA graph capture

Note: PR #34179 is adding V2 runner support for DCP with full CUDA graphs. Once merged, this check may become unnecessary, but it's critical for current users.

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[gemini-code-assist]

Code Review

This pull request correctly re-adds a critical compatibility check for DCP/PCP with full CUDA graphs on the CUDA platform, which was inadvertently removed. The change prevents crashes by overriding the cudagraph_mode to PIECEWISE when these features are enabled with full CUDA graphs. I've suggested a minor refactoring to improve code maintainability by removing a duplicated line.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Siritao.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork