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

astrai/inference/__init__.py

@@ -17,7 +17,7 @@ from astrai.inference.api import (
     MessagesRequest,
     ProtocolHandler,
     StopChecker,
-    app,
+    get_app,
     run_server,
 )
 from astrai.inference.api.anthropic import AnthropicResponseBuilder
@@ -80,6 +80,6 @@ __all__ = [
     "ChatCompletionRequest",
     "AnthropicMessage",
     "MessagesRequest",
-    "app",
+    "get_app",
     "run_server",
 ]