[HiCache] Count hit allocations and in-flight backups in the buffer pipeline idle check

[xiezhq-hermann]

Motivation

BufferModePipeline.is_idle() tells the cache when buffer-mode transfer work is quiescent, which gates the idle-time memory invariant check. It missed two kinds of in-flight work: storage hits parked in pending_hit_allocs waiting for host staging (they still hold prefetch occupancy and restart I/O once space frees), and nodes in inflight_backup_node_ids whose backup intent was admitted but whose D2H staging has not launched. Either can make the idle checker run against a pipeline that is about to move memory and report a spurious leak.

Modifications

• is_idle() now also returns False while pending_hit_allocs or inflight_backup_node_ids are non-empty. The docstring is updated to match.

Original commits

• 5e155a3447

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.
• Please feel free to join our Slack channel at https://slack.sglang.io to discuss your PR.

Testing

• ruff format --check, ruff check --select F401,F821,UP037, py_compile on the changed file.
• Buffer-mode / staged-prefetch / anchor-lock slice of test_unified_radix_cache_unittest.py and test_hicache_staged_write_back_dispatch.py with SGLANG_ENABLE_ASYNC_ASSERT=true: 127 passed, 279 skipped (non-applicable component configs).
• Validated end-to-end on an internal L3-backed buffer-mode CI lane with the strict idle memory check enabled; the spurious idle-leak reports no longer occur. Not re-run from this branch; relying on CI for GPU suites.

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CI States

Latest PR Test (Base): 🚫 Run #33815929441
Latest PR Test (Extra): ❌ Run #33815929214
Latest PR Test (AMD ROCm 7.2): 🚫 Run #33815929395

[xiezhq-hermann]

/tag-and-rerun-ci