fix(ds4): harden sparse verifier metadata for DCP1 and padded replays

[voipmonitor]

Problem

Two metadata edge cases could feed invalid sparse-attention state into DS4 verification:

1. DCP=1 reconstructed causal lengths from padded capacity instead of exact GPU positions.
2. Padded CUDA-graph rows reused a live request slot, so stale request metadata could affect global top-k/cache indexing.

Both are correctness risks under long-context or capacity-padded replays and were previously mixed into the large #88 stack.

Fix

• Preserve and consume exact GPU causal lengths for the DCP1 path.
• Reserve a dedicated safe padding request slot and ensure stale padding indices cannot enter global top-k selection.
• Keep TP/DCP metadata construction deterministic across replay shapes.

Validation

• test_global_topk_ignores_stale_padding_request_index
• DCP variant of the stale-padding test
• exact GPU-position/causal-length test
• Result: 3 passed on SM120
• ruff check, ruff format --check, and git diff --check

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Review details

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Configuration used: Organization UI

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Run ID: cedf21e4-4189-4439-ad1f-7d75b2ef776e

📥 Commits

Reviewing files that changed from the base of the PR and between e4879e1 and 458a946.

📒 Files selected for processing (4)

• tests/models/deepseek_v4/test_cache_utils.py
• tests/v1/attention/test_sparse_mla_backends.py
• vllm/models/deepseek_v4/common/ops/cache_utils.py
• vllm/v1/attention/backends/mla/b12x_mla_sparse.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/ff-ds4-sparse-metadata-safety-20260717

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