[v0.18.0][CI]Add rank0 process count check for DeepSeek-R1-W8A8-HBM test (#8072)

### What this PR does / why we need it?
Adds a `check_rank0_process_count` validation step to the
DeepSeek-R1-W8A8-HBM nightly single-node test.

The check verifies that after the server starts, there is **exactly 1**
`vllm serve` process running on rank0. This guards against the
regression fixed in #8041 (extra NPU context leaking on device 0),
ensuring it does not silently reappear in future releases.

#### Changes

-
**`tests/e2e/nightly/single_node/models/scripts/test_single_node.py`**:
Add `run_check_rank0_process_count` async handler. It calls `npu-smi
info` for diagnostics, then uses `psutil` to assert exactly one `vllm
serve` process exists on rank0.
-
**`tests/e2e/nightly/single_node/models/configs/DeepSeek-R1-W8A8-HBM.yaml`**:
Register `check_rank0_process_count` in the `test_content` list for the
HBM test case.

Signed-off-by: hfadzxy <starmoon_zhang@163.com>

tests/e2e/nightly/single_node/models/configs/DeepSeek-R1-W8A8-HBM.yaml

@@ -39,4 +39,7 @@ test_cases:
       - "--enforce-eager"
       - "--additional-config"
       - '{"ascend_scheduler_config": {"enabled": false}, "torchair_graph_config": {"enabled": false, "enable_multistream_shared_expert": false}}'
+    test_content:
+      - completion
+      - check_rank0_process_count
     benchmarks: