feat: 完成 issue #59 ⑥ 昇腾 NPU 后端适配（CANN 对接）

core/inference-backend/factory.py

@@ -40,14 +40,18 @@ def build_backend(config: dict) -> InferenceBackend:
         )
     cls = BACKEND_REGISTRY[name]
     inf = config.get("inference", {}) or {}
-    return cls(
+    kwargs = dict(
         endpoint=inf.get("endpoint", ""),
         model=inf.get("model", "iaop-default"),
         timeout_seconds=float(inf.get("timeout_seconds", 10)),
-        runtime=inf.get("runtime", ""),
-        device=inf.get("device", ""),
-        cann_version=inf.get("cann_version", ""),
     )
+    # 空值/缺省不覆盖后端类默认（如昇腾 npu 默认 mindie / ascend-910b / CANN 8.0），
+    # 保证「切换后端仅改 backend 字段」时硬件相关参数落到正确的默认值。
+    for key in ("runtime", "device", "cann_version"):
+        value = inf.get(key)
+        if value:
+            kwargs[key] = value
+    return cls(**kwargs)
 
 
 def load_backend_config(path: str) -> dict: