[CH] findTopLevelSortMergeJoinTransform never matches on ClickHouse, so the migrated SPARK-37652 skew-join case is ignored on 3.4+

### Description

`ClickHouseAdaptiveQueryExecSuite` carries a Gluten rewrite of `SPARK-37652: optimize skewed join through union`. It was migrated from `gluten-ut/spark33` to the 3.4 / 3.5 / 4.0 / 4.1 modules in #12837, and it is committed there as `ignoreGluten` because it fails on ClickHouse 3.5.

The immediate failure is that `findTopLevelSortMergeJoinTransform` collects zero joins where the case expects two:

```
- Gluten - SPARK-37652: optimize skewed join through union *** FAILED ***
List() had size 0 instead of expected size 2 (ClickHouseAdaptiveQueryExecSuite.scala:1597)
```

This is a type mismatch rather than a plan-shape difference. The helper matches `org.apache.gluten.execution.SortMergeJoinExecTransformer`, which is the concrete Velox case class (`SortMergeJoinExecTransformer.scala:230`). ClickHouse plans these joins as `CHSortMergeJoinExecTransformer` or `CHShuffledHashJoinExecTransformer`, which extend `SortMergeJoinExecTransformerBase` (`SortMergeJoinExecTransformer.scala:37`) and `ShuffledHashJoinExecTransformerBase` respectively, and neither is a subtype of the Velox class. So on ClickHouse the helper returns an empty result for any query.

A backend-neutral type is available: `ColumnarShuffledJoin` (`JoinExecTransformer.scala:42`) declares `isSkewJoin` and is mixed into both `MergeJoinLikeExecTransformer` and `ShuffledHashJoinExecTransformerBase`, so `collect(plan) { case j: ColumnarShuffledJoin => j }` covers both backends and both join shapes.

That alone may not be enough to re-enable the case. The expected counts (2/2, 1/1, 3/0) are copied verbatim from spark33 and have never been checked against ClickHouse 3.5, so someone with a ClickHouse build needs to confirm or correct them.

Two related observations for whoever picks this up:

`findTopLevelSortMergeJoinTransform` has eight other call sites in the same suite, and they are all subject to the same mismatch on ClickHouse.

The neighbouring skew-join case in that suite already has its `checkSkewJoin(inner, 2, 1)` calls commented out (`ClickHouseAdaptiveQueryExecSuite.scala:707`), which suggests this area was parked earlier for what may be the same reason.

Fixing the helper touches shared test infrastructure and is out of scope for #12837, which is a pure test migration.

This description was written with AI assistance (Claude Code).

### Gluten version

main branch

Start with findTopLevelSortMergeJoinTransform and its eight call sites in ClickHouseAdaptiveQueryExecSuite.scala, then inspect the join transformer types in JoinExecTransformer.scala and SortMergeJoinExecTransformer.scala. Run the migrated SPARK-37652 case against ClickHouse 3.5 and compare the expected counts with the observed plans. Done means the helper covers the intended ClickHouse joins and the case is re-enabled with validated expectations.

Written by the indexing model from the issue text.