fix: default reasoning_effort to 'none' for new assistants

解决Issue #11919: Cerebras AI GLM 4.6推理选项问题

问题: 新建助手时reasoning_effort为undefined，与UI显示的"关闭"语义不一致
解决: 在DEFAULT_ASSISTANT_SETTINGS中初始化reasoning_effort为'none'

影响:
- 所有新建助手的默认推理设置
- 符合UI显示语义和用户期望
- 通用于所有推理模型，不仅仅是Cerebras

文件:
- src/renderer/src/services/AssistantService.ts: 添加reasoning_effort默认值

src/renderer/src/services/AssistantService.ts

@@ -38,7 +38,9 @@ export const DEFAULT_ASSISTANT_SETTINGS = {
   enableTopP: false,
   // It would gracefully fallback to prompt if not supported by model.
   toolUseMode: 'function',
-  customParameters: []
+  customParameters: [],
+  // Default reasoning_effort to 'none'
+  reasoning_effort: 'none'
 } as const satisfies AssistantSettings
 
 export function getDefaultAssistant(): Assistant {