fix: accept nullable teacher default reasoning effort

electron/coding-teacher/model-runner.ts

@@ -39,11 +39,16 @@ export async function prepareTeacherModel(account: TeacherAccount, definition: T
       '老师所用模型暂不可用，请联系运营调整。'
     );
   }
+  const savedChoice = definition.model.reasoning_choice;
+  // Published definitions serialize an unspecified effort as null.
+  const choice = savedChoice.mode === 'enabled'
+    ? { mode: savedChoice.mode, ...(savedChoice.effort == null ? {} : { effort: savedChoice.effort }) }
+    : { mode: savedChoice.mode };
   let fields: Record<string, unknown>;
   try {
     fields = buildManagedModelRequest(
       modelId,
-      definition.model.reasoning_choice,
+      choice,
       capability
     ).reasoningFields;
   } catch {