refactor : FastAPI 懒加载单例，消除模块级副作用

- import astrai.inference 不再在模块加载时创建 FastAPI 实例
- 路由移至 APIRouter；get_app() 首次调用时懒构造单例
- _create_engine 和 run_server 的 param_path 改为必填
- 更新测试改用 get_app() 替代模块级 app

tests/inference/conftest.py

@@ -5,21 +5,22 @@ from unittest.mock import MagicMock
 import pytest
 from fastapi.testclient import TestClient
 
-from astrai.inference import app
+from astrai.inference import get_app
 
 
 @pytest.fixture
 def client():
     """Provide a test client for the FastAPI app."""
-    app.state.server_config = {
+    _app = get_app()
+    _app.state.server_config = {
         "device": "cpu",
         "dtype": "bfloat16",
         "param_path": None,
         "max_batch_size": 1,
         "_test": True,
     }
-    app.state.engine = None
-    return TestClient(app)
+    _app.state.engine = None
+    return TestClient(_app)
 
 
 @pytest.fixture
@@ -49,5 +50,5 @@ def mock_engine():
 @pytest.fixture
 def loaded_model(client, mock_engine):
     """Simulate that the engine is loaded."""
-    app.state.engine = mock_engine
+    get_app().state.engine = mock_engine
     return mock_engine