docs: 补全租户与字典模块注释

src/Application/TakeoutSaaS.Application/App/Tenants/Handlers/CreateTenantBillingCommandHandler.cs

@@ -14,6 +14,12 @@ namespace TakeoutSaaS.Application.App.Tenants.Handlers;
 public sealed class CreateTenantBillingCommandHandler(ITenantBillingRepository billingRepository)
     : IRequestHandler<CreateTenantBillingCommand, TenantBillingDto>
 {
+    /// <summary>
+    /// 处理创建租户账单请求。
+    /// </summary>
+    /// <param name="request">创建命令。</param>
+    /// <param name="cancellationToken">取消标记。</param>
+    /// <returns>账单 DTO。</returns>
     public async Task<TenantBillingDto> Handle(CreateTenantBillingCommand request, CancellationToken cancellationToken)
     {
         // 1. 校验账单编号