### 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>
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