docs: 补全租户与字典模块注释

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

@@ -12,6 +12,12 @@ namespace TakeoutSaaS.Application.App.Tenants.Handlers;
 public sealed class MarkTenantBillingPaidCommandHandler(ITenantBillingRepository billingRepository)
     : IRequestHandler<MarkTenantBillingPaidCommand, TenantBillingDto?>
 {
+    /// <summary>
+    /// 标记账单支付。
+    /// </summary>
+    /// <param name="request">标记命令。</param>
+    /// <param name="cancellationToken">取消标记。</param>
+    /// <returns>账单 DTO 或 null。</returns>
     public async Task<TenantBillingDto?> Handle(MarkTenantBillingPaidCommand request, CancellationToken cancellationToken)
     {
         // 1. 查询账单