[SPARK-50749][SQL] Fix ordering bug in CommutativeExpression.gatherCommutative method

[utkarsh39]

What changes were proposed in this pull request?

SPARK-49977 introduced a bug in the CommutativeExpression.gatherCommutative() method, changing the function's output order.
Consider the following concrete example:

val addExpression = Add(
  Literal(1),
  Add(
    Literal(2),
    Literal(3)
  )
)
val commutativeExpressions = addExpression.gatherCommutative(addExpression,
  { case Add(l, r, _) => Seq(l, r)})

Consider the output of the gatherCommutative method. SPARK-49977 introduced a bug that reversed the output order. This PR fixes the bug in gatherCommutative() to restore the original correct ordered output.

// Prior to [SPARK-49977](https://issues.apache.org/jira/browse/SPARK-49977) and after this fix
// commutativeExpressions -> Seq(Literal(1), Literal(2), Literal(3)))
// Post [SPARK-49977](https://issues.apache.org/jira/browse/SPARK-49977) and before this fix
// commutativeExpressions -> Seq(Literal(3), Literal(2), Literal(1)))

Why are the changes needed?

Fixing a bug

Does this PR introduce any user-facing change?

No

How was this patch tested?

Added a test

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

Generated-by: ChatGPT

[cloud-fan]

The first commit passed CI (spark connect failure unrelated), and the new commit just moved the test. I'm merging it to master, thanks!