Fix NTFS filename character sanitization on non-Windows systems

Replace OS-specific Path.GetInvalidFileNameChars() with a comprehensive
list of characters invalid on common filesystems (NTFS, FAT32, ext4, etc.)
to ensure files can be created on any filesystem regardless of host OS.

- Add InvalidFileNameChars array with all NTFS/Windows forbidden chars
- Add comprehensive unit tests for filename sanitization
- Fixes issue where '?', ':', '*', '<', '>', '|', '"', '\' were not sanitized on Linux

Co-authored-by: Tyrrrz <1935960+Tyrrrz@users.noreply.github.com>

DiscordChatExporter.Cli.Tests/Specs/PathSanitizationSpecs.cs

@@ -0,0 +1,129 @@
+using System.IO;
+using DiscordChatExporter.Core.Utils.Extensions;
+using FluentAssertions;
+using Xunit;
+
+namespace DiscordChatExporter.Cli.Tests.Specs;
+
+public class PathSanitizationSpecs
+{
+    [Fact]
+    public void Path_with_question_mark_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("How to do this?");
+
+        // Assert
+        result.Should().Be("How to do this_");
+    }
+
+    [Fact]
+    public void Path_with_colon_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("Title: Subtitle");
+
+        // Assert
+        result.Should().Be("Title_ Subtitle");
+    }
+
+    [Fact]
+    public void Path_with_asterisk_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("File*Name");
+
+        // Assert
+        result.Should().Be("File_Name");
+    }
+
+    [Fact]
+    public void Path_with_backslash_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("Path\\File");
+
+        // Assert
+        result.Should().Be("Path_File");
+    }
+
+    [Fact]
+    public void Path_with_forward_slash_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("Path/File");
+
+        // Assert
+        result.Should().Be("Path_File");
+    }
+
+    [Fact]
+    public void Path_with_double_quote_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("Say \"Hello\"");
+
+        // Assert
+        result.Should().Be("Say _Hello_");
+    }
+
+    [Fact]
+    public void Path_with_less_than_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("Value<10");
+
+        // Assert
+        result.Should().Be("Value_10");
+    }
+
+    [Fact]
+    public void Path_with_greater_than_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("Value>10");
+
+        // Assert
+        result.Should().Be("Value_10");
+    }
+
+    [Fact]
+    public void Path_with_pipe_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("Option A | Option B");
+
+        // Assert
+        result.Should().Be("Option A _ Option B");
+    }
+
+    [Fact]
+    public void Path_with_multiple_invalid_characters_is_sanitized()
+    {
+        // Act
+        var result = Path.EscapeFileName("How? Why: This/That <works> |right|");
+
+        // Assert
+        result.Should().Be("How_ Why_ This_That _works_ _right_");
+    }
+
+    [Fact]
+    public void Path_with_valid_characters_is_not_changed()
+    {
+        // Act
+        var result = Path.EscapeFileName("Valid File Name 123");
+
+        // Assert
+        result.Should().Be("Valid File Name 123");
+    }
+
+    [Fact]
+    public void Path_with_special_but_valid_characters_is_not_changed()
+    {
+        // Act
+        var result = Path.EscapeFileName("File (with) [brackets] & symbols! @#$%^");
+
+        // Assert
+        result.Should().Be("File (with) [brackets] & symbols! @#$%^");
+    }
+}

DiscordChatExporter.Core/Utils/Extensions/PathExtensions.cs

@@ -1,11 +1,31 @@
 using System;
 using System.IO;
+using System.Linq;
 using System.Text;
 
 namespace DiscordChatExporter.Core.Utils.Extensions;
 
 public static class PathExtensions
 {
+    // Characters that are invalid on common filesystems.
+    // This is a union of invalid characters from Windows (NTFS/FAT32), Linux (ext4/XFS), and macOS (HFS+/APFS).
+    // We use this instead of Path.GetInvalidFileNameChars() because that only returns OS-specific characters,
+    // not filesystem-specific characters. For example, on Linux, '?' is valid in filenames, but not on NTFS.
+    // https://github.com/Tyrrrz/DiscordChatExporter/issues/1417
+    private static readonly char[] InvalidFileNameChars =
+    [
+        '\0', // Null character - invalid on all filesystems
+        '/', // Path separator on Unix
+        '\\', // Path separator on Windows
+        ':', // Reserved on Windows (drive letters, NTFS streams)
+        '*', // Wildcard on Windows
+        '?', // Wildcard on Windows
+        '"', // Reserved on Windows
+        '<', // Redirection on Windows
+        '>', // Redirection on Windows
+        '|', // Pipe on Windows
+    ];
+
     extension(Path)
     {
         public static string EscapeFileName(string path)
@@ -13,7 +33,7 @@ public static class PathExtensions
             var buffer = new StringBuilder(path.Length);
 
             foreach (var c in path)
-                buffer.Append(!Path.GetInvalidFileNameChars().Contains(c) ? c : '_');
+                buffer.Append(!InvalidFileNameChars.Contains(c) ? c : '_');
 
             // File names cannot end with a dot on Windows
             // https://github.com/Tyrrrz/DiscordChatExporter/issues/977