[Bugfix] Fix GDN attention crash with mixed decode/spec-decode batches (#34871)

Signed-off-by: haosdent <haosdent@gmail.com>

vllm/v1/attention/backends/gdn_attn.py

@@ -220,6 +220,16 @@ class GDNAttentionMetadataBuilder(AttentionMetadataBuilder[GDNAttentionMetadata]
                 query_lens_cpu.sum().item() - num_prefill_tokens - num_decode_tokens
             )
 
+            # num_decodes and num_spec_decodes are mutually exclusive.
+            # Reclassify non-spec decodes as prefills when spec decodes
+            # exist — the prefill kernel handles 1-token sequences with
+            # initial state correctly, producing identical results.
+            if num_decodes > 0 and num_spec_decodes > 0:
+                num_prefills += num_decodes
+                num_prefill_tokens += num_decode_tokens
+                num_decodes = 0
+                num_decode_tokens = 0
+
             if num_prefills == 0 and num_decodes == 0:
                 spec_token_size = min(
                     num_spec_decodes * (self.num_spec + 1),