[HiCache] Resolve write_back to write_through under buffer_only host memory

[metamergebot]

Motivation

With --hicache-host-memory-mode buffer_only the host pool is only a staging buffer: every cached page lives in the storage backend, so there is no retained host tier for --hicache-write-policy write_back to defer writes into. Argument resolution rejected the pair at startup. That turns a launch config which carries write_back as its general HiCache policy into a failed launch the moment it switches the host memory mode, even though the only policy buffer-only can run is already determined by the mode. The same file already resolves other incompatible HiCache pairs (layout vs. I/O backend, storage backend vs. layout) instead of rejecting them.

Modifications

• validate_hicache_host_memory_mode resolves write_back to write_through under buffer_only through declare_resolution, with an info log naming the rewrite, instead of raising. The rewrite is that narrow: an explicit write_through_selective stays as given, and the cache mode keeps whatever policy it was given.
• TestHiCacheArgs.test_buffer_only_resolves_write_back_to_write_through pins the resolved policy for each write policy under buffer_only and for write_back under cache. The write_back case fails on the previous code with the old ValueError.

The HiCache hook already runs before the optimistic-prefill gate in the resolution pipeline, so that gate reads the resolved write_through. No documentation describes the rejection, so none changes.

Ran locally: test/registered/unit/server_args/test_server_args.py (TestHiCacheArgs), test/registered/unit/server_args/test_resolution_declarations.py and test/registered/unit/mem_cache/test_decode_retraction_backup.py, all passing; the pinned pre-commit linters on both files.

Accuracy Tests

Not applicable: argument resolution only, no change to model forward or kernels.

Speed Tests and Profiling

Not applicable.

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.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.

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CI States

Latest PR Test (Base): 🚫 Run #37452625943
Latest PR Test (Extra): ⚠️ Not enabled -- add run-ci-extra label to opt in.
Latest PR Test (AMD ROCm 10): ❌ Run #37452626037