fix(IE.Excel): 图片下载改 per-call HttpClient 修并发回归

f27f28f 把 WebClient 换成共享 static HttpClient 后,跨 class 并行测试
并发下载偶发失败,异常被上层 catch 吞掉导致 drawing 缺失。改为 per-call
using new HttpClient,30s 超时保留。

Author: hueifeng

src/Magicodes.ExporterAndImporter.Excel/Images/ImageExtensions.cs

@@ -9,11 +9,6 @@ namespace Magicodes.IE.Excel.Images
 {
     public static partial class ImageExtensions
     {
-        private static readonly HttpClient _httpClient = new HttpClient
-        {
-            Timeout = TimeSpan.FromSeconds(30)
-        };
-
         public struct ImageInfo
         {
             public int Width;
@@ -42,7 +37,11 @@ public static ImageInfo IdentifyImage(byte[] imageBytes)
 
         public static byte[] DownloadImageBytes(this string url)
         {
-            return _httpClient.GetByteArrayAsync(url).ConfigureAwait(false).GetAwaiter().GetResult();
+            // per-call 实例：避免共享 static HttpClient 在并发下载下偶发失败导致图片静默丢失
+            using (var http = new HttpClient { Timeout = TimeSpan.FromSeconds(30) })
+            {
+                return http.GetByteArrayAsync(url).ConfigureAwait(false).GetAwaiter().GetResult();
+            }
         }
 
         public static byte[] ReadImageBytes(this string filePath)
@@ -73,7 +72,8 @@ private static string GetContentType(SKEncodedImageFormat format)
 
         public static SixLabors.ImageSharp.Image GetImageByUrl(this string url, out SixLabors.ImageSharp.Formats.IImageFormat format)
         {
-            using (Stream webStream = _httpClient.GetStreamAsync(url).ConfigureAwait(false).GetAwaiter().GetResult())
+            using (var http = new HttpClient { Timeout = TimeSpan.FromSeconds(30) })
+            using (Stream webStream = http.GetStreamAsync(url).ConfigureAwait(false).GetAwaiter().GetResult())
             {
                 using (MemoryStream memoryStream = new MemoryStream())
                 {