[Spec Decode] fix returning size mismatch on extract hidden states proposer (#38610)

Signed-off-by: Jaebok Lee <jaebok9541@naver.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

vllm/v1/spec_decode/extract_hidden_states.py

@@ -145,7 +145,10 @@ class ExtractHiddenStatesProposer:
 
         # Return the sampled tokens as "draft" tokens
         # Shape: [batch_size, 1] to match num_speculative_tokens=1
-        return sampled_token_ids
+        # On decode steps with spec tokens, sampled_token_ids may have
+        # shape [batch_size, 2] (target + spec verification); slice to
+        # return only the target-sampled column.
+        return sampled_token_ids[:, :1]
 
     def _get_slot_mapping(
         self,