[Prefix Cache] Support fine-grained SWA hits

[RichApple123]

Purpose

Fixes #53786 by allowing SlidingWindowManager to participate in fine-grained hybrid prefix-cache lookup when its physical KV page is larger than the configured hash unit.

The worker-facing block table remains physical-page based. This change only makes prefix endpoints hash-granular:

• register the producer's last prompt boundary inside an SWA page through the existing BlockPool.cache_partial_block primitive;
• scan sparse fine-boundary endpoints right-to-left, apply an EAGLE/MTP drop of one hash unit, and validate every physical page intersecting the resulting sliding window;
• return null-padded physical block lists, preserving global page indices;
• retain the exact reachable physical span under sparse retention, including the speculative endpoint page when EAGLE/MTP proves a boundary across a page;
• reuse the existing partial-hit copy-on-write path when resuming inside an SWA page;
• if fine cross-manager reconciliation loses a usable Mamba/EAGLE checkpoint,
  retry at the scheduler-page boundary and keep the longer reconciled result.

This is deliberately narrower than the adjacent work:

• [Spec Decode][Core] Run an all-sliding DFlash drafter with prefix caching enabled #50457 represents an all-sliding DFlash group as full attention when prefix caching is enabled; this PR supports fine lookup in SlidingWindowManager itself.
• [Bugfix][Spec Decode] Cache the Mamba state at the block-grid position of EAGLE resume #53945/[Kimi K3] Support internal prefix checkpoints with partial prefix caching and spec-decoding #53614 add Mamba resume checkpoints. They may still be required for a hybrid common hit to survive fixed-point reconciliation, but are not SWA page-table changes.
• EAGLE/MTP prefix-cache last-block drop causes a 1,648-token recompute per hit on one hybrid Qwen3.8 GDN layout — ~30-40% batch throughput loss on prefix-reusing workloads with speculative decoding #53670 and the DFlash/DSpark classification changes in [Bugfix][Spec Decode] Restore hybrid mamba align cache hits under spec decode with a KV connector (related to #53505) #54165 concern whether/where the speculative last unit is dropped. This PR preserves the drop and reduces its unit to the shared hash boundary for fine-capable SWA.

No kernel/page-alignment workaround is needed: all returned block IDs and CoW
copies remain physical pages. The coarse fallback preserves checkpoint + tail
replay when Mamba state cannot resume at a fine boundary; internal Mamba
checkpoints remain separate follow-up work.

AI assistance was used to inspect related work, draft the implementation and tests, and prepare this description. The contributor must review every changed line and run the reported commands before submission, per AGENTS.md.

Test Plan

uvx ruff format --check \
  vllm/v1/core/kv_cache_coordinator.py \
  vllm/v1/core/single_type_kv_cache_manager.py \
  tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py
uvx ruff check \
  vllm/v1/core/kv_cache_coordinator.py \
  vllm/v1/core/single_type_kv_cache_manager.py \
  tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py
.venv/bin/python -m pytest \
  tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py \
  tests/v1/core/test_single_type_kv_cache_manager.py \
  tests/v1/core/test_prefix_caching.py -q
.venv/bin/python -m pytest tests/v1/core -q

Single-H20 validation performed:

• cold producer vs identical warm consumer at several prompt lengths;
• deterministic output-token comparison and cached-token/TTFT measurements;
• manager/CUDA smoke coverage for interior-page eviction, sparse retention,
  physical-page validation, and copy-on-write;
• resolved geometry: hash unit 16, SWA physical page 816 tokens, window 2048,
  with the target's auto-aligned Mamba checkpoint geometry.

Multi-GPU TP/DCP/PCP geometry is follow-up validation and is not inferred from a TP=1 run.

Test Result

• Focused manager/prefix suite: 146 passed.
• Ruff format/check and git diff --check: passed.
• Full tests/v1/core source-only run: 562 passed, with one failure and
  five collection/setup errors. Three fixture errors pass when run with the
  repository root conftest.py (3 passed); the remaining failure and two
  errors are CPU engine E2E tests that cannot initialize because this checkout
  has no compiled vllm._C extension (init_cpu_memory_env is unavailable).
  None reaches the changed manager code.
• Single-H20 manager/CUDA smoke: passed on vLLM 0.28.0+cu129 and
  PyTorch 2.13.0+cu129. This covered fine lookup, physical span validation,
  EAGLE cross-page fallback, interior-page eviction, sparse retention, and the
  actual CUDA physical-page CoW operation.
• Single-H20 serving completed with a Qwen3.8 FP8 target and a five-layer
  DFlash SWA draft geometry. The unpatched coarse path hit 4,896/6K tokens;
  fine-only reconciliation regressed to zero; the final fallback restored the
  4,896-token hit and matching output hash. This confirms that Mamba internal
  checkpoints are still required for finer end-to-end EAGLE hits.

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