文件名清理改为固定的 Windows 非法字符集，Linux CI 现在也会将 : 替换为 -。

清除了 DOCX 测试的空引用警告，以及 3 处 xUnit Assert.Single 分析器警告。

src/Jiaowu.Api/Infrastructure/Files/DownloadFileName.cs

@@ -5,6 +5,11 @@ namespace Jiaowu.Api.Infrastructure.Files;
 /// </summary>
 public static class DownloadFileName
 {
+    // Do not use Path.GetInvalidFileNameChars(): its result is platform-specific,
+    // while downloaded names must remain usable on Windows regardless of where
+    // the API is built or hosted.
+    private static readonly char[] WindowsInvalidFileNameChars = ['<', '>', ':', '"', '/', '\\', '|', '?', '*'];
+
     public static string Export(string title, string extension, params string?[] parts) =>
         Build(title, extension, includeTimestamp: true, parts);
 
@@ -33,9 +38,8 @@ public static class DownloadFileName
     private static string Clean(string? value)
     {
         if (string.IsNullOrWhiteSpace(value)) return string.Empty;
-        var invalid = Path.GetInvalidFileNameChars();
         var cleaned = new string(value.Trim().Select(character =>
-            invalid.Contains(character) ? '-' : character).ToArray());
+            WindowsInvalidFileNameChars.Contains(character) || char.IsControl(character) ? '-' : character).ToArray());
         return cleaned.Trim('.', ' ');
     }