[Unified Tree] fix: exempt host-locked aux nodes from the sanity_check host-LRU check

[alphabetc1]

Motivation

sanity_check raises while a storage-backup or load-back host lock is in flight.

The aux (SWA / Mamba) host-lock acquire delists its node from the host LRU on the
0 -> 1 transition and relists it on -> 0, so the eviction walker cannot pick a
node whose host slice is being read. sanity_check did not know about that: it
required every host-only aux node to be in the host LRU, so any outstanding host
lock was reported as <component> host LRU mismatch.

SGLANG_ENABLE_TREE_CACHE_SANITY_CHECK defaults to on in CI, and
InvariantChecker._check_tree_cache runs it for hybrid-SWA / hybrid-SSM models, so
a HiCache backup in flight when the checker fires turns into an intermittent CI
failure that is not a product defect.

Modifications

Exempt nodes with host_lock_ref > 0 from the aux host-LRU membership comparison in
both tree cores. The exemption is two-sided on purpose: acquire delists a node, but a
node locked while already listed stays listed (eviction relists it at the device
tombstone), so both states are legal for a locked node.

The unlocked invariant is unchanged and still enforced -- an unlocked host-only value
missing from the LRU is still a violation
(sanity_check_detects_a_host_only_value_missing_from_the_lru).

sanity_check_detects_a_host_locked_value_missing_from_the_lru asserted the behaviour
being removed, so it becomes the positive case and now shares the fixture helper with
its two siblings. No production behaviour changes; this is a diagnostic-only fix.

Accuracy Tests

• cargo test --locked --lib: 867 passed, 0 failed, 1 ignored.
• test_unified_radix_cache_unittest.py: 1256 passed, 1435 skipped, 70 subtests.
• Targeted repro on one H200, driving a real UnifiedRadixCache at the tree API:
  sanity_check passes before, during and after an outstanding storage-backup host
  lock (it raised during the lock before this change).
• Randomized lock-ledger stress (insert / match / split / lock / early SWA release /
  release / evict), 6 seeds x 150 steps for full, full,swa and full,swa,mamba:
  0 invariant violations in all three.

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.

🤖 Generated with Claude Code

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CI States

Latest PR Test (Base): ✅ Run #35295275230
Latest PR Test (Extra): ✅ Run #35295274985
Latest PR Test (AMD ROCm 10): ❌ Run #35295275311

[alphabetc1]

/rerun-group radix_cache/unified_radix_tree

[github-actions]

Results for /rerun-group radix_cache/unified_radix_tree:

🚀 4-gpu-h100 (5 tests): ❌ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/linker/test_unified_cache_linker_kl_dsv4.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_hicache_pp_kl.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_cp.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dsv4.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_mamba.py

🚀 8-gpu-h200 (4 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/linker/test_unified_cache_linker_kl_glm52.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dsv4_pp.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_glm52.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_mimo.py

🚀 4-gpu-b200 (1 test): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_dcp.py

🚀 2-gpu-h100 (2 tests): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_full.py
cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_swa.py

🚀 1-gpu-h100 (1 test): ✅ View workflow run

cd test/ && python3 registered/radix_cache/unified_radix_tree/test_unified_radix_cache_kl_hybrid_bitexact.py