Fix UTF-8 BOM handling in MonacoPreviewHandler to prevent crashes

[Copilot]

Issue

PowerToys.MonacoPreviewHandler crashes when trying to render files with UTF-8 BOM (Byte Order Mark). This issue affects users who want to preview text files with UTF-8 BOM encoding in File Explorer's preview pane.

Root cause

When reading a file with UTF-8 BOM using StreamReader:

1. In some cases, the BOM character (\uFEFF) can remain in the string content after ReadToEnd()
2. When this content is later re-encoded to UTF-8 during base64 conversion:

   _base64FileCode = Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(fileContent));

3. The incorrectly handled BOM causes issues during rendering, leading to a crash

Solution

The fix adds an explicit check to detect and remove the BOM character after reading the file content:

// Remove BOM character if present to avoid encoding issues when converting to base64
// BOM character might be present as U+FEFF even though StreamReader should normally handle it
if (fileContent.Length > 0 && fileContent[0] == '\uFEFF')
{
    fileContent = fileContent.Substring(1);
    Logger.LogInfo("Removed UTF-8 BOM character from file content");
}

This ensures that any UTF-8 BOM is properly handled before base64 conversion, preventing the crash while maintaining correct rendering of the file content.

Fixes #16.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• cdn.fwupd.org
  • Triggering command: /usr/bin/fwupdmgr refresh (dns block)
• crl3.digicert.com
  • Triggering command: dotnet build src/modules/previewpane/MonacoPreviewHandler/MonacoPreviewHandler.csproj (dns block)
• ocsp.digicert.com
  • Triggering command: dotnet build src/modules/previewpane/MonacoPreviewHandler/MonacoPreviewHandler.csproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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