[SM120][Bugfix] Fix packed KV block zeroing stride

[wangxian001]

Purpose

Apply the packed KV cache zeroing fix to ds4-sm120-preview-dev. The upstream counterpart is vllm-project#50276.

The zeroing path currently uses a segment's page size for both its logical block stride and its zero span. Packed DeepSeek KV views can have a larger physical stride than an individual segment's page, so clearing a high block ID may touch adjacent packed data or cross the end of the allocation.

This change:

• tracks logical block stride independently from page size;
• represents virtual kernel blocks as independent zeroing segments;
• derives the zero span from the tensor shape and strides; and
• adds a final-block regression test for a packed backing allocation.

The SM120 prefill/decode paths, DeepSeek-V4 kernels, and DSpark speculative decoding behavior are unchanged.

Duplicate-work check

The matching upstream fix is vllm-project#50276. Searches of open JASL issues and PRs for KVBlockZeroer and packed KV zeroing found no other implementation on this branch.

Test Plan

• .venv/Scripts/python.exe -m py_compile vllm/v1/worker/utils.py tests/v1/worker/test_kv_block_zeroer.py
• ruff 0.14.0 check vllm/v1/worker/utils.py tests/v1/worker/test_kv_block_zeroer.py
• ruff 0.14.0 format --check vllm/v1/worker/utils.py tests/v1/worker/test_kv_block_zeroer.py
• pytest -q tests/v1/worker/test_kv_block_zeroer.py on CUDA
• End-to-end DeepSeek-V4 serving with FP8 KV cache, block size 256, prefix caching, DSpark speculative decoding, and mixed long-context refill and short structured requests

Test Result

• Python 3.12 syntax checks passed.
• Ruff lint and format checks passed.
• Targeted CUDA regression suite: 3 passed.
• End-to-end validation passed on two RTX PRO 6000 Blackwell GPUs under repeated cache pressure; the API remained healthy and no CUDA Xid was observed after the fix.

AI assistance disclosure: OpenAI Codex helped prepare the patch and PR text. The submitter participated in the diagnosis, reviewed the change, and validated it end-to-end.

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