fix: copy spec_info in ScheduleBatch.copy() to preserve Eagle hidden states in overlap prefill

In disaggregated prefill with --enable-single-batch-overlap,
batch.copy() was used to defer process_batch_result, but the copy
omitted spec_info. This caused req.hidden_states_tensor to be None
when using Eagle/spec-v2 speculative decoding on the decode server,
silently degrading draft quality for the first decode round.

python/sglang/srt/managers/schedule_batch.py

@@ -2343,6 +2343,7 @@ class ScheduleBatch(ScheduleBatchDisaggregationDecodeMixin):
             return_logprob=self.return_logprob,
             decoding_reqs=self.decoding_reqs,
             spec_algorithm=self.spec_algorithm,
+            spec_info=self.spec_info,
             global_num_tokens=self.global_num_tokens,
             global_num_tokens_for_logprob=self.global_num_tokens_for_logprob,
             can_run_dp_cuda_graph=self.can_run_dp_cuda_graph,