Handle dynamic offset index after adding columns in tiering IT

Author: buvb

fluss-lake/fluss-lake-paimon/src/test/java/org/apache/fluss/lake/paimon/tiering/PaimonTieringITCase.java

@@ -560,8 +560,9 @@ private void checkDataInPaimonAppendOnlyTable(
             InternalRow flussRow = flussRowIterator.next();
             assertThat(row.getInt(0)).isEqualTo(flussRow.getInt(0));
             assertThat(row.getString(1).toString()).isEqualTo(flussRow.getString(1).toString());
-            // the idx 2 is __bucket, so use 3
-            assertThat(row.getLong(3)).isEqualTo(startingOffset++);
+            // system columns are always the last three: __bucket, __offset, __timestamp
+            int offsetIndex = row.getFieldCount() - 2;
+            assertThat(row.getLong(offsetIndex)).isEqualTo(startingOffset++);
         }
         assertThat(flussRowIterator.hasNext()).isFalse();
     }