test: port mixed_chunked_prefill and load_snapshot_server to NPU

[hhhh1252023]

Port two low-difficulty CUDA test files to Ascend NPU.

• test_npu_mixed_chunked_prefill.py: verifies --enable-mixed-chunk with/without radix cache, GSM8K accuracy threshold 0.1 for Llama-3.2-1B
• test_npu_load_snapshot_server.py: verifies /v1/loads endpoint with [no-dp, dp=2] x [shm, zmq] backends

CI triggered via single-test-npu.yml.

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