Add DataRowExtensions for safe value retrieval with defaults

Introduced DataRowExtensions.cs with extension methods for DataRow:
- ItemEx<T>: Retrieves a value by column name with a default if missing or null.
- ItemEx (string): Overload for string values, using the generic method.
These methods help prevent errors when accessing missing or null columns.

EnvelopeGenerator.ServiceHost/Extensions/DataRowExtensions.cs

@@ -0,0 +1,27 @@
+using System.Data;
+
+namespace EnvelopeGenerator.ServiceHost.Extensions;
+
+public static class DataRowExtensions
+{
+    public static T ItemEx<T>(this DataRow row, string columnName, T defaultValue)
+    {
+        if (!row.Table.Columns.Contains(columnName))
+        {
+            return defaultValue;
+        }
+
+        var value = row[columnName];
+        if (value is DBNull or null)
+        {
+            return defaultValue;
+        }
+
+        return (T)Convert.ChangeType(value, typeof(T));
+    }
+
+    public static string ItemEx(this DataRow row, string columnName, string defaultValue)
+    {
+        return row.ItemEx<string>(columnName, defaultValue);
+    }
+}