[Bugfix] fix pcp + eplb error (#5561)

### What this PR does / why we need it?
Fix the bug in the PCP overlay feature

1、Fix the bug related to PCP and EPLB overlap by including PCP size in
the word_size calculation.
2、In the PCP pooling scenario, a prompt has been added for setting the
cp_kv_cache_interleave_size.

- vLLM version: v0.13.0
- vLLM main:
7157596103

Signed-off-by: weiguihua2 <weiguihua2@huawei.com>

tests/e2e/nightly/multi_node/config/Qwen3-235B-W8A8-longseq.yaml

@@ -9,6 +9,7 @@ env_common:
   HCCL_BUFFSIZE: 1024
   SERVER_PORT: 8080
   NUMEXPR_MAX_THREADS: 128
+  DYNAMIC_EPLB: true
 disaggregated_prefill:
   enabled: true
   prefiller_host_index: [0]
@@ -52,6 +53,8 @@ deployment:
                   }
             }
         }'
+        --additional-config
+          '{"dynamic_eplb":true}'
 
   -
     server_cmd: >
@@ -90,4 +93,6 @@ deployment:
                   }
             }
         }'
+        --additional-config
+          '{"dynamic_eplb":true}'
 benchmarks: