docs: 完善微信商户平台模块的示例项目

samples/SKIT.FlurlHttpClient.Wechat.TenpayV3.Sample_Net5/Services/BackgroundServices/WxpayCertificateRefreshingBackgroundService.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using SKIT.FlurlHttpClient.Wechat.TenpayV3;
+using SKIT.FlurlHttpClient.Wechat.TenpayV3.Models;
+using SKIT.FlurlHttpClient.Wechat.TenpayV3.Settings;
+
+namespace SKIT.FlurlHttpClient.Wechat.TenpayV3.Sample_Net5.Services.BackgroundServices
+{
+    class WxpayCertificateRefreshingBackgroundService : BackgroundService
+    {
+        private readonly ILogger _logger;
+        private readonly Options.WxpayOptions _wxpayOptions;
+        private readonly CertificateManager _certificateManager;
+        private readonly HttpClients.IWechatTenpayHttpClientFactory _wechatTenpayHttpClientFactory;
+
+        public WxpayCertificateRefreshingBackgroundService(
+            ILoggerFactory loggerFactory,
+            IOptions<Options.WxpayOptions> wxpayOptions,
+            CertificateManager certificateManager,
+            HttpClients.IWechatTenpayHttpClientFactory wechatTenpayHttpClientFactory)
+        {
+            _logger = loggerFactory.CreateLogger(GetType());
+            _wxpayOptions = wxpayOptions.Value;
+            _certificateManager = certificateManager;
+            _wechatTenpayHttpClientFactory = wechatTenpayHttpClientFactory;
+        }
+
+        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+        {
+            while (!stoppingToken.IsCancellationRequested)
+            {
+                var wxpayMerchant = _wxpayOptions.Merchants.FirstOrDefault();
+                if (wxpayMerchant == null)
+                {
+                    _logger.LogWarning("未找到微信商户配置项。");
+                    break;
+                }
+
+                try
+                {
+                    var client = _wechatTenpayHttpClientFactory.Create(wxpayMerchant.MerchantId);
+                    var request = new QueryCertificatesRequest();
+                    var response = await client.ExecuteQueryCertificatesAsync(request, cancellationToken: stoppingToken);
+                    if (response.IsSuccessful())
+                    {
+                        client.DecryptResponseEncryptedData(ref response);
+                        foreach (var cert in response.CertificateList)
+                        {
+                            _certificateManager.SetCertificate(cert.SerialNumber, cert.EncryptCertificate.CipherText);
+                        }
+
+                        _logger.LogInformation("刷新微信商户平台证书成功。");
+                    }
+                    else
+                    {
+                        _logger.LogWarning(
+                            "刷新微信商户平台证书失败（状态码：{0}，错误代码：{1}，错误描述：{2}）。",
+                            response.RawStatus, response.ErrorCode, response.ErrorMessage
+                        );
+                    }
+                }
+                catch (Exception ex)
+                {
+                    _logger.LogError(ex, "刷新微信商户平台证书遇到异常。");
+                }
+
+                await Task.Delay(TimeSpan.FromDays(1)); // 每隔 1 天轮询刷新
+            }
+        }
+    }
+}