[None][fix] Preserve reusable KV cache prefix snapshots

[jiaganc]

@coderabbitai summary

Description

Per-request block reuse can remove a shorter partial endpoint when a longer sibling covers the same token prefix, even when the longer sibling does not yet preserve all lifecycle state needed to reuse that endpoint. With commit_min_snapshot, a later rewind can then follow an exact radix-tree path whose pages are not reusable and miss an older reusable sibling prefix.

This change:

• keeps covered partial endpoints until the covering block has committed pages for every lifecycle, including an SSM snapshot at the same token boundary;
• removes redundant covered siblings after the replacement state has been committed;
• evaluates partial sibling matches by reusable tokens after lifecycle pruning; and
• falls back from an unusable exact path to the best reusable sibling prefix.

There is no public API change.

Test Coverage

• Added test_commit_min_snapshot_preserves_partial_endpoint to cover preservation and reuse of a rewind endpoint with mixed full-attention and SWA lifecycles.
• pre-commit run --from-ref upstream/main --to-ref HEAD passes.
• python3 -m py_compile tensorrt_llm/runtime/kv_cache_manager_v2/_block_radix_tree.py passes.

Known validation gap: the latest AGA concurrency-144 end-to-end reproduction still reported 21 low-hit rewind requests with zero cached tokens. This PR remains a draft while that discrepancy is investigated.

PR Checklist

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[jiaganc]

Close. Will be replaced by @lowsfer another PR.