修复腾讯财经接口代码大小写问题：改为大写

AssetManager.Infrastructure/Services/MarketDataService.cs

@@ -49,8 +49,8 @@ public class MarketDataService : IMarketDataService
         {
             _logger.LogInformation($"Requesting stock price for symbol: {symbol} (腾讯财经接口)");
 
-            // 腾讯财经美股接口：前缀us，小写代码
-            var url = $"http://qt.gtimg.cn/q=us{symbol.ToLower()}";
+            // 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);
 
             if (string.IsNullOrEmpty(response) || !response.Contains("~"))