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/src/JdeScoping.DataSync/Etl/Transformers/DataTransformerBase.cs

@@ -62,4 +62,13 @@ public abstract class DataTransformerBase : IDataTransformer
     /// Override to handle null transformations.
     /// </summary>
     public virtual bool IsDBNull(int ordinal, IDataReader source) => source.IsDBNull(ordinal);
+
+    /// <summary>
+    /// Maps a transformed ordinal to the source ordinal.
+    /// Override to support column reordering or computed columns.
+    /// </summary>
+    /// <param name="transformedOrdinal">The ordinal in the transformed output.</param>
+    /// <param name="source">The source data reader.</param>
+    /// <returns>The corresponding source ordinal, or -1 for computed columns.</returns>
+    public virtual int MapOrdinal(int transformedOrdinal, IDataReader source) => transformedOrdinal;
 }