修复腾讯财经接口GBK编码报错问题：注册编码提供程序，手动解析GBK响应

AssetManager.Infrastructure/Services/MarketDataService.cs

@@ -1,6 +1,7 @@
 using System.Net.Http.Json;
 using System.Security.Cryptography;
 using System.Text;
+using System.Text.RegularExpressions;
 using AssetManager.Data;
 using AssetManager.Models.DTOs;
 using Microsoft.Extensions.Logging;
@@ -49,9 +50,13 @@ public class MarketDataService : IMarketDataService
         {
             _logger.LogInformation($"Requesting stock price for symbol: {symbol} (腾讯财经接口)");
 
+            // 注册GBK编码支持
+            Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+            
             // 腾讯财经美股接口：前缀us，大写代码
             var url = $"http://qt.gtimg.cn/q=us{symbol.ToUpper()}";
-            var response = await _httpClient.GetStringAsync(url);
+            var responseBytes = await _httpClient.GetByteArrayAsync(url);
+            var response = Encoding.GetEncoding("GBK").GetString(responseBytes);
 
             if (string.IsNullOrEmpty(response) || !response.Contains("~"))
             {