Fix SM120 NVFP4 KV cache test OOM

[Fridge003]

Summary

• move the SM120 Llama NVFP4 KV-cache test from models_e2e to quant
• use GSM8KMixin with DefaultServerBase for one server configuration instead of sweeping through run_combined_tests
• keep the test registered in extra-a on the 1-gpu-small runner
• lower mem_fraction_static from the default 0.885 to 0.87, preserving the test's 200-request concurrency while leaving about 0.47 GiB more runtime headroom on a 32 GiB RTX 5090
• temporarily disable the registration because the test is currently failing its accuracy threshold

The test only covers the NVFP4 GEMM + NVFP4 KV-cache configuration; it does not add an MTP variant.

Root cause

The test maximizes its NVFP4 KV-cache allocation under the default static-memory fraction. During the first ModelOpt FP4 GEMM in FlashInfer autotuning, CUDA then has only 28.44 MiB free when FlashInfer requests another 32 MiB, so server startup fails and the test harness later reports exit code -9.

This is a narrow CI containment for the newly added test; it does not change production allocator behavior. The test remains disabled until its separate accuracy failure is resolved.

Failing job: https://github.com/sgl-project/sglang/actions/runs/29624032862/job/88035809117

Validation

• pre-commit run --files test/registered/quant/test_llama8b_nvfp4_kv_cache_sm120.py
• git diff --check
• verified the CI registration checker accepts the Extra registration and disabled reason

The full end-to-end test requires an SM120 GPU and is intentionally not scheduled while disabled.

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CI States

Latest PR Test (Base): ❌ Run #29636016544
Latest PR Test (Extra): ❌ Run #29636016460

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[Fridge003]

/rerun-test test/registered/models_e2e/test_llama8b_nvfp4_kv_cache_sm120.py

[github-actions]

Results for /rerun-test test/registered/models_e2e/test_llama8b_nvfp4_kv_cache_sm120.py:

🚀 1-gpu-5090 (1 test): ❌ View workflow run

cd test/ && python3 registered/models_e2e/test_llama8b_nvfp4_kv_cache_sm120.py

[Fridge003]

/rerun-test test/registered/quant/test_llama8b_nvfp4_kv_cache_sm120.py

[github-actions]

Results for /rerun-test test/registered/quant/test_llama8b_nvfp4_kv_cache_sm120.py:

🚀 1-gpu-5090 (1 test): ❌ View workflow run

cd test/ && python3 registered/quant/test_llama8b_nvfp4_kv_cache_sm120.py