[Fix] Preserve model runner contracts in prefill CUDA graphs

[aurickq]

Motivation

PrefillCudaGraphRunner constructs new ForwardBatch objects for capture and replay. Those objects currently bypass ModelRunner.prepare_dummy_forward_batch, even though model runners use that hook to prepare graph-specific batch state. The runner also rebuilds LogitsProcessorOutput while trimming graph padding but drops customized_info.

Modifications

• Apply prepare_dummy_forward_batch to both capture-time and replay-time ForwardBatch objects.
• Preserve customized_info when trimming full-prefill graph outputs.
• Add focused tests for both contracts.

Accuracy Tests

No model math or kernels are changed.

• 7 focused unit tests passed, including 2 parameterized subtests.

Speed Tests and Profiling

Not applicable. The hooks run during graph setup/replay preparation, and the output change only preserves existing metadata.

Checklist

• Format code with the repository pre-commit configuration.
• Add unit tests.
• No documentation update is required.
• No speed benchmark is required for this contract-preservation fix.
• Follow the SGLang code style guidance.

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CI States

Latest PR Test (Base): ❌ Run #35505146165
Latest PR Test (Extra): ❌ Run #35505146680
Latest PR Test (AMD ROCm 10): ❌ Run #35505145958

[aurickq]

/tag-and-rerun-ci

[ispobock]

/rerun-failed-ci

[Oasis-Git]

LGTM

[Oasis-Git]

Extra Pass: https://github.com/sgl-project/sglang/actions/runs/32770385571

[Oasis-Git]

Based failed on known ci issue: https://github.com/sgl-project/sglang/actions/runs/32770385846/job/97633999335?pr=35452

[Oasis-Git]

/rerun-group cpu

[github-actions]

Results for /rerun-group cpu:

🚀 ubuntu-latest (34 tests): ❌ View workflow run

cd test/ && python3 registered/cpu/arm64/test_moe.py
cd test/ && python3 registered/cpu/quant/test_autoround.py
cd test/ && python3 registered/cpu/test_activation.py
cd test/ && python3 registered/cpu/test_binding.py
cd test/ && python3 registered/cpu/test_bmm.py
cd test/ && python3 registered/cpu/test_causal_conv1d.py
cd test/ && python3 registered/cpu/test_comm.py
cd test/ && python3 registered/cpu/test_cpu_graph.py
cd test/ && python3 registered/cpu/test_decode.py
cd test/ && python3 registered/cpu/test_extend.py
cd test/ && python3 registered/cpu/test_flash_attn.py
cd test/ && python3 registered/cpu/test_gemm.py
cd test/ && python3 registered/cpu/test_intel_amx_attention_backend_a.py
cd test/ && python3 registered/cpu/test_intel_amx_attention_backend_b.py
cd test/ && python3 registered/cpu/test_intel_amx_attention_backend_c.py
cd test/ && python3 registered/cpu/test_mamba.py
cd test/ && python3 registered/cpu/test_mla.py
cd test/ && python3 registered/cpu/test_moe.py
cd test/ && python3 registered/cpu/test_norm.py
cd test/ && python3 registered/cpu/test_qkv_proj_with_rope.py
cd test/ && python3 registered/cpu/test_qwen3.py
cd test/ && python3 registered/cpu/test_rank_consensus_checker.py
cd test/ && python3 registered/cpu/test_request_decompression.py
cd test/ && python3 registered/cpu/test_request_headers.py
cd test/ && python3 registered/cpu/test_rope.py
cd test/ && python3 registered/cpu/test_server_args_backend.py
cd test/ && python3 registered/cpu/test_shared_expert.py
cd test/ && python3 registered/cpu/test_spec_eagle_cpu.py
cd test/ && python3 registered/cpu/test_spec_eagle_parity_cpu.py
cd test/ && python3 registered/cpu/test_spec_eagle_topk_cpu.py
cd test/ && python3 registered/cpu/test_spec_kernels.py
cd test/ && python3 registered/cpu/test_store_cache.py
cd test/ && python3 registered/cpu/test_subblock_sparse_attention.py
cd test/ && python3 registered/cpu/test_topk.py

[Oasis-Git]

/rerun-failed-ci

[Oasis-Git]

/rerun-failed-ci

[ispobock]

/rerun-failed-ci

[Oasis-Git]

base pass: https://github.com/sgl-project/sglang/actions/runs/35400403299.