test(agg): drop brittle memory-delta assert on mixed-schema path

Addresses @kosiew's review on #23523. The assert
`column_size < rows_size` couples this correctness test to Arrow's Row
memory accounting; the printed comparison stays for reviewer
visibility. Memory delta is more appropriately tracked via
benchmarks.

Author: zhuqi-lucas

datafusion/physical-plan/src/aggregates/group_values/multi_group_by/mod.rs

@@ -1367,8 +1367,13 @@ mod tests {
         assert_eq!(g1, g2, "group assignment must match the rows fallback");
 
         // (b) Memory: the column-wise path stores the 8 native columns compactly
-        //     and only row-encodes the nested one, so it must be smaller than
+        //     and only row-encodes the nested one, so it should be smaller than
         //     encoding every column into rows.
+        //
+        // The delta is only printed here — a hard `column_size < rows_size`
+        // assert would be brittle to future Arrow row-format or memory-
+        // accounting changes without reflecting a grouping-correctness
+        // regression. Track the memory improvement via benchmarks instead.
         let column_size = column_path.size();
         let rows_size = rows_path.size();
         println!(
@@ -1377,11 +1382,6 @@ mod tests {
              ({:.1}% of fallback)",
             100.0 * column_size as f64 / rows_size as f64
         );
-        assert!(
-            column_size < rows_size,
-            "expected column-wise path ({column_size}) to use less memory than \
-             the all-rows fallback ({rows_size})"
-        );
 
         // Emitted values must be equal too (compare via the rows fallback which
         // is the established reference implementation).