feat: add validation feature for row count comparison

Add a validate command that compares row counts between source and target tables after a migration.

internal/app/etl/extractors/process.go

@@ -76,6 +76,7 @@ func (ex *GenericExtractor) ProcessPartition(
 
 	batchRows := make([]models.UnknownRowValues, 0, batchSize)
 	var rowsRead int64 = 0
+	var lastRow models.UnknownRowValues
 
 	for rows.Next() {
 		rowValues := make([]any, len(columns))
@@ -90,7 +91,7 @@ func (ex *GenericExtractor) ProcessPartition(
 				return rowsRead, err
 			}
 
-			if err := flush(ctx, &partition, batchSize, batchRows, chBatchesOut); err != nil {
+			if err := flush(ctx, batchSize, batchRows, chBatchesOut); err != nil {
 				return rowsRead, err
 			}
 
@@ -98,11 +99,12 @@ func (ex *GenericExtractor) ProcessPartition(
 			return rowsRead, errorFromLastPartitionRow(lastRow, indexPrimaryKey, partition, err)
 		}
 		rowsRead++
+		lastRow = rowValues
 
 		batchRows = append(batchRows, rowValues)
 		if len(batchRows) >= batchSize {
 			// logrus.Debugf("Batch size reached, flushing batch with %v rows (rowsRead=%v)", len(batchRows), rowsRead)
-			if err := flush(ctx, &partition, batchSize, batchRows, chBatchesOut); err != nil {
+			if err := flush(ctx, batchSize, batchRows, chBatchesOut); err != nil {
 				// logrus.Warnf("Error flushing rows: %v", err)
 				return rowsRead, err
 			}
@@ -110,9 +112,16 @@ func (ex *GenericExtractor) ProcessPartition(
 		}
 	}
 
-	if err := flush(ctx, &partition, batchSize, batchRows, chBatchesOut); err != nil {
+	if err := flush(ctx, batchSize, batchRows, chBatchesOut); err != nil {
 		return rowsRead, err
 	}
 
-	return rowsRead, rows.Err()
+	if err := rows.Err(); err != nil {
+		if lastRow != nil {
+			return rowsRead, errorFromLastPartitionRow(lastRow, indexPrimaryKey, partition, err)
+		}
+		return rowsRead, err
+	}
+
+	return rowsRead, nil
 }