fix: 优化微信授权服务的用户创建逻辑

移除冗余的用户ID赋值步骤，直接使用插入后返回的主键值填充日志，简化代码逻辑

QYZH.InteractiveMagazine.Service/WeChatAuthService.cs

@@ -260,11 +260,10 @@ public class WeChatAuthService(
             UpdatedAt = DateTime.Now
         };
 
-        var userId = await wxUserRepository.Context.Insertable(newUser).ExecuteReturnIdentityAsync();
-        newUser.Id = userId;
+        await wxUserRepository.Context.Insertable(newUser).ExecuteReturnIdentityAsync();
 
         logger.LogInformation("新用户创建成功，UserId: {UserId}, WxUserId: {WxUserId}, Name: {Name}",
-            userId, wxUserId, input.Name);
+            newUser.Id, wxUserId, input.Name);
 
         // 为新用户创建默认宠物
         try { await petService.CreateDefaultPetAsync(newUser.Id); }