Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Infrastructure/Services/StringExtensions.cs

@@ -0,0 +1,16 @@
+using System.Xml.Serialization;
+
+namespace DocumentService.Infrastructure.Services;
+
+public static class StringExtensions
+{
+    public static T? DeserializeXml<T>(this string xmlText)
+    {
+        if (string.IsNullOrWhiteSpace(xmlText))
+            return default;
+
+        var serializer = new XmlSerializer(typeof(T));
+        using var reader = new StringReader(xmlText);
+        return (T?)serializer.Deserialize(reader);
+    }
+}