[SPARK-54905][SQL] Simplify foreachWithSubqueries implementation in QueryPlan

### What changes were proposed in this pull request?

This PR simplifies the foreachWithSubqueries method in QueryPlan.scala by:
1. Directly traversing both subqueries and children using the same traverse function
2. Replacing the indirect foreach(actualFunc) call with a direct traverse(this) call

### Why are the changes needed?

The original implementation was unnecessarily complex, using foreach to traverse children implicitly while recursively calling foreachWithSubqueries for subqueries. The new implementation is simpler and more straightforward.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing tests for query plan traversal should cover this change.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Cursor 2.2.44

Closes #53681 from heyihong/SPARK-54905.

Authored-by: Yihong He <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: heyihong

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/QueryPlan.scala

@@ -623,11 +623,9 @@ abstract class QueryPlan[PlanType <: QueryPlan[PlanType]]
    * A variant of [[foreach]] which considers plan nodes inside subqueries as well.
    */
   def foreachWithSubqueries(f: PlanType => Unit): Unit = {
-    def actualFunc(plan: PlanType): Unit = {
-      f(plan)
-      plan.subqueries.foreach(_.foreachWithSubqueries(f))
-    }
-    foreach(actualFunc)
+    f(this)
+    subqueries.foreach(_.foreachWithSubqueries(f))
+    children.foreach(_.foreachWithSubqueries(f))
   }
 
   /**