【main】【bugfix】fix: restrict default MLAPO activation to Decode nodes only (#6451)

### What this PR does / why we need it?
There is an issue with the current default logic for MLAPO (MLA Policy
Optimization). By design, MLAPO should only be enabled by default on
Decode (D) nodes. However, in hybrid (collocated prefill and decode)
scenarios, the strategy is erroneously activated during the Prefill
stage.
This PR corrects the default behavior to ensure that MLAPO is
exclusively enabled for the Decoding phase. This prevents unexpected
policy interference during Prefill and ensures optimal performance in
hybrid deployment environments.
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.14.1
- vLLM main:
dc917cceb8

---------

Signed-off-by: fems14 <1804143737@qq.com>

vllm_ascend/attention/mla_v1.py

@@ -22,7 +22,7 @@ from vllm_ascend.attention.utils import (
     AscendCommonAttentionMetadata,
     ascend_chunked_prefill_workspace_size,
     enable_cp,
-    enabling_malpo,
+    enabling_mlapo,
     maybe_save_kv_layer_to_connector,
     split_decodes_and_prefills,
     trans_rope_weight,
@@ -710,7 +710,7 @@ class AscendMLAImpl(MLAAttentionImpl):
         self.ring_mla_mask_size = 512
 
         self.speculative_config = self.vllm_config.speculative_config
-        self.enable_mlapo = enabling_malpo(self.vllm_config)
+        self.enable_mlapo = enabling_mlapo(self.vllm_config)
 
         self.is_kv_producer = (
             self.vllm_config.kv_transfer_config is not None and self.vllm_config.kv_transfer_config.is_kv_producer