Allow platforms to declare Mamba extra-buffer support

[xuzijian629]

Motivation

Mamba extra-buffer validation currently uses a fixed list of device types. This prevents out-of-tree platforms from enabling the feature even when they implement the required cache-state snapshot behavior, and it couples server argument validation to specific hardware names.

Modifications

• Add SRTPlatform.support_mamba_cache_extra_buffer() as the platform capability used by server argument validation.
• Preserve the existing behavior for CUDA, ROCm, MUSA, NPU, and XPU platforms.
• Allow other platforms to opt in by overriding the capability method.
• Add CPU unit tests covering the default capability behavior, an out-of-tree override, and server argument validation dispatch.

Accuracy Tests

N/A. This changes platform capability validation only and does not modify model computation or numerical behavior.

Speed Tests and Profiling

N/A. This change is outside the inference hot path.

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations. N/A: the new platform capability is documented inline and does not change user-facing configuration.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed. N/A: no model computation or inference hot path is changed.
• Follow the SGLang code style guidance.

Review and Merge Process

1. Ping Merge Oncalls to start the process. See the PR Merge Process.
2. Get approvals from CODEOWNERS and other reviewers.
3. Trigger CI tests with comments or contact authorized users to do so.
   • Common commands include /tag-and-rerun-ci, /tag-run-ci-label, /rerun-failed-ci
4. After green CI and required approvals, ask Merge Oncalls or people with Write permission to merge the PR.

---

CI States

Latest PR Test (Base): ❌ Run #37397056745
Latest PR Test (Extra): ❌ Run #37397056334
Latest PR Test (AMD ROCm 10): ❌ Run #37397056631

[alexnails]

/tag-and-rerun-ci

[BBuf]

Thanks! This branch now needs to be updated onto the latest main. The Mamba extra-buffer validation has moved from ServerArgs._validate_mamba_extra_buffer to python/sglang/srt/arg_groups/mamba_hook.py::validate_mamba_extra_buffer. Please port the current_platform.support_mamba_cache_extra_buffer() check and the corresponding test/patch target to that hook, then re-request review.

[xuzijian629]

@BBuf @alexnails thanks for reviewing! Rebased onto master

[xuzijian629]

/tag-and-rerun-ci

[xuzijian629]

@alexnails thanks for reviewing this PR!! I rebased the branch. Are we ready to merge it?

[xuzijian629]

All PR Test Base tests are passing, except for the H200 DeepSeek-v4 FlashInfer/EAGLE ones, which are unrelated to my change.
Our code only runs for Mamba extra-buffer validation, which this DeepSeek test never reaches.