[Bugfix][KV Offloading] Preserve MTP/Mamba state across external hits

[Leonccaa]

Purpose

Fix incorrect native CPU KV offload restores for hybrid Mamba/GDN models when
MTP speculative decoding is enabled.

This PR depends on 1Cat PR #247. PR #247 removes the original external-KV
assertion and backports the Mamba boundary-alignment fixes from upstream vLLM,
but it does not cover MTP-owned accepted-token state or mutable Mamba align
source blocks. This branch is stacked on the fixed head of #247
(c514f39e15ca74a1f51bb5270f0f4734d1da8bc7) and will be rebased after #247
lands.

Root cause

On a real external CPU-tier hit, MTP4 output consistently lost exactly four
tokens. The fixed loss matching the MTP width exposed two related ownership
bugs:

1. Asynchronous accepted-token D2H copies targeted mutable InputBatch rows.
   Rows can be removed, condensed, resumed, or reused before the event is
   consumed, so accepted-token state could be applied to the wrong request.
2. A Mamba align table is not append-only. MTP scratch blocks move to new
   logical positions when a boundary is committed, while the native connector's
   block mirror only appends. A positional store could therefore save a stale
   Mamba state under a valid prefix key.

Changes

• Store accepted-token D2H results in runner-owned buffers and remap them by
  request identity after synchronization. New, resumed, or same-ID replacement
  requests reset their speculative state.
• Clip attention block tables at the computed-token frontier without clipping
  allocator-defined non-attention state groups.
• Emit exact core-selected Mamba align boundary block IDs to the connector.
• Exclude mutable Mamba groups from the normal positional store path and build
  their store jobs only from exact boundary handoffs.
• Keep the existing pending-job fence on each source block until the async
  store completes, including deduplication when MTP aliases one physical block.

Scope and duplicate-work check

This is not duplicate work:

• 1Cat [Bugfix][KV Offloading] Backport mamba/GDN CPU offloading fixes (upstream vllm#42554 + vllm#44599) #247 is the required base fix; it does not address MTP state ownership
  or the exact external-hit corruption described here.
• Upstream vLLM
  #51358 implements exact
  Mamba boundary persistence for Mooncake with its own pin/watermark protocol;
  this PR adapts the correctness invariant to the native OffloadingConnector
  and its per-job fence lifecycle.
• Upstream vLLM
  #52771 fixes MTP/EAGLE
  lookups being reduced to zero. In this reproduction the external hit was
  already real; the restored recurrent state was wrong.
• Upstream vLLM
  #52832 concerns
  Mooncake finish-time partial tails and does not cover this native exact-
  boundary path.

No open 1Cat PR or issue was found for this mechanism.

Test Plan

.venv/bin/python -m pytest -q \
  tests/v1/core/test_prefix_caching.py \
  tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py \
  tests/v1/core/test_scheduler.py

.venv/bin/python -m pytest -q tests/v1/worker/test_gpu_model_runner.py \
  -k 'sync_mamba_accepted_token_state_tracks_request_ownership or mamba_align_postprocess_uses_runner_owned_d2h_buffers or mamba_align_cpu_postprocess_mirrors_runner_owned_snapshot or mamba_all_mode_uses_runner_owned_d2h_buffers'

pre-commit run --from-ref upstream/pr-247 --to-ref HEAD
git diff --check upstream/pr-247..HEAD

Test Result

• Core, scheduler, and native connector regression suite: 219 passed.
• New worker state-ownership tests: 4 passed.
• All pre-commit hooks for the ten changed files passed.
• git diff --check passed.
• An extended run including the entire worker file produced 251 passed, 3
  failed. All three failures reproduce unchanged on the unmodified [Bugfix][KV Offloading] Backport mamba/GDN CPU offloading fixes (upstream vllm#42554 + vllm#44599) #247 base:
  one existing sampling mock lacks drafter, and two GPU/FLASHINFER assertions
  are not valid in the CPU-only test environment.
• Integrated 4 x Tesla V100 validation of the identical MTP diff, together with
  [Bugfix][KV Offloading] Backport mamba/GDN CPU offloading fixes (upstream vllm#42554 + vllm#44599) #247 and the separately submitted VMM-safe connector change in 1Cat [Bugfix][KV Offloading] Allow native CPU offload with expandable_segments #248,
  passed a
  39,999-token cold / external-hit / local-rehit sequence with byte- and
  SHA-identical output. Exact-boundary, mid-block, Responses API, tool-call, and
  follow-up cases passed; MTP reported 136 drafted / 135 accepted tokens, with
  no OOM, process restart, or EngineCore fatal error.

AI assistance

AI assistance was used for implementation, test orchestration, root-cause
analysis, and drafting this PR. The human submitter has reviewed every changed line and is submitting this
PR for maintainer review.

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[yangzhuxinyzx]

源码审计通过：accepted-token D2H 状态按请求身份安全重映射；attention lookahead 裁剪与非 attention 状态隔离；Mamba align 使用同一步 scheduler 的精确 boundary block handoff，并在异步 store 生命周期沿用 source-block fence。当前 main 合并态及 diff/语法/Ruff/格式检查通过。