feat: handle invalid xml characters

[NikiforovAll]

This pull request includes several changes to the Clippit.Tests/Excel/Samples/SpreadsheetWriterSamples.cs and Clippit/Excel/SpreadsheetWriter.cs files to improve code readability, initialization, and XML handling. The most important changes include the removal of unnecessary using statements, the conversion of array initializations to collection initializations, and the addition of a method to sanitize XML strings.

Code readability and initialization improvements:

• Clippit.Tests/Excel/Samples/SpreadsheetWriterSamples.cs: Removed unnecessary using statements and converted array initializations to collection initializations in Sample1 and Sample2 methods. [1] [2] [3] [4] [5] [6]

XML handling improvements:

• Clippit/Excel/SpreadsheetWriter.cs: Removed unnecessary using statements and added using System.Globalization and using System.Text.
• Clippit/Excel/SpreadsheetWriter.cs: Initialized Worksheets property with an empty collection in WorkbookDfn class.
• Clippit/Excel/SpreadsheetWriter.cs: Modified date and time formatting to use CultureInfo.InvariantCulture in the out int numColumns method.
• Clippit/Excel/SpreadsheetWriter.cs: Added SanitizeXmlString method to handle invalid XML characters.

[sergey-tihon]

1. Please take a look at failed build
2. Copilot found typos in the code and propose some nice suggestions

3. Why do we need to introduce option and choose between exception and removal? Why we cannot properly encode invalid symbols and keep them in Excel?

private static string SanitizeXmlString(string? xml)
{
    if (string.IsNullOrEmpty(xml))
    {
        return string.Empty;
    }

    var buffer = new StringBuilder(xml.Length);

    foreach (var c in xml)
    {
        if (XmlConvert.IsXmlChar(c))
        {
            buffer.Append(c);
        }
        else
        {
            buffer.AppendFormat("&#x{0:X};", (int)c); // Encode invalid characters
        }
    }

    return buffer.ToString();
}

[NikiforovAll]

How did you generate copilot suggestions?

[sergey-tihon]

How did you generate copilot suggestions?

I have Copilot integrated into GitHub
https://docs.github.com/en/enterprise-cloud@latest/copilot/using-github-copilot/asking-github-copilot-questions-in-github

[NikiforovAll]

I actually, have it too. Thank you for the tip