[DSA] Re-enable fused top-k v2 for MTP: clamp padded-row seq_lens to >= 0 (#30378)

Co-authored-by: ziyi.xu <ziyi.xu@radixark.ai>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

test/registered/kernels/test_dsa_indexer.py

@@ -640,6 +640,12 @@ class TestDSAIndexer(CustomTestCase):
             )
         ).contiguous()
 
+        # The fused v2 PAGED dispatch requires the per-forward plan to be
+        # preprocessed alongside the metadata (it asserts rather than silently
+        # recomputing it) -- mirror what init_forward_metadata /
+        # _build_forward_metadata_cuda_graph do.
+        from sglang.jit_kernel.dsv4.topk import plan_topk_v2
+
         attn_metadata = DSAMetadata(
             page_size=1,
             cache_seqlens_int32=seq_lens_expanded.clone(),
@@ -654,6 +660,7 @@ class TestDSAIndexer(CustomTestCase):
             dsa_cu_seqlens_k=dsa_cu_seqlens_k,
             dsa_extend_seq_lens_list=seq_lens_expanded.cpu().tolist(),
             dsa_seqlens_expanded=seq_lens_expanded,
+            topk_v2_plan=plan_topk_v2(seq_lens_expanded),
             topk_indices_offset=(
                 topk_indices_offset
                 if topk_transform_method == TopkTransformMethod.RAGGED