feat(etl): add MapOrdinal to IDataTransformer interface

Add MapOrdinal method to the IDataTransformer interface and provide
a default implementation in DataTransformerBase. This enables
transformers to report the mapping between transformed ordinals and
source ordinals, supporting use cases like computed columns which
return -1 to indicate no source ordinal.

- Add MapOrdinal(int, IDataReader) to IDataTransformer interface
- Add virtual MapOrdinal implementation in DataTransformerBase
- Add DataTransformerBaseTests with test for default behavior

NEW/tests/JdeScoping.DataSync.Tests/Etl/Transformers/DataTransformerBaseTests.cs

@@ -0,0 +1,40 @@
+using System.Data;
+using JdeScoping.DataSync.Etl.Transformers;
+using NSubstitute;
+
+namespace JdeScoping.DataSync.Tests.Etl.Transformers;
+
+public class DataTransformerBaseTests
+{
+    [Fact]
+    public void MapOrdinal_DefaultImplementation_ReturnsOrdinalUnchanged()
+    {
+        // Arrange
+        var transformer = new PassThroughTransformer();
+        var reader = CreateMockReader(new[] { "A", "B", "C" });
+
+        // Act
+        var result = transformer.MapOrdinal(1, reader);
+
+        // Assert
+        Assert.Equal(1, result);
+    }
+
+    private static IDataReader CreateMockReader(string[] columns)
+    {
+        var reader = Substitute.For<IDataReader>();
+        reader.FieldCount.Returns(columns.Length);
+        for (int i = 0; i < columns.Length; i++)
+        {
+            var index = i;
+            reader.GetName(index).Returns(columns[index]);
+            reader.GetOrdinal(columns[index]).Returns(index);
+        }
+        return reader;
+    }
+
+    private class PassThroughTransformer : DataTransformerBase
+    {
+        public override string TransformerName => "PassThrough";
+    }
+}