[Feat] Adapted mtp function to Qwen3-next (#3918)

### What this PR does / why we need it?

Adapts mtp function to Qwen3-next.

- vLLM version: v0.11.0
- vLLM main:
83f478bb19

Signed-off-by: drslark <slarksblood@qq.com>

tests/ut/attention/test_attention_v1.py

@@ -77,6 +77,7 @@ class TestAscendAttentionMetadataBuilder(TestBase):
         mock_get_dcp_group.return_value = dcp_group
 
         self.mock_vllm_config = MagicMock()
+        self.mock_vllm_config.speculative_config = None
         self.mock_vllm_config.model_config.max_model_len = 640
         self.mock_vllm_config.cache_config.block_size = 64
         self.mock_vllm_config.compilation_config.cudagraph_mode = None