array_union result ordering versus DataFusion is unverified #4681

Describe the bug

The array expression audit (#4483) noted that array_union result ordering versus DataFusion is unverified. Spark's array_union preserves left-array elements first, followed by elements from the right array that are not already present. CometArrayUnion (spark/src/main/scala/org/apache/comet/serde/arrays.scala) is currently registered as Compatible, so if DataFusion's array_union does not preserve that ordering, Comet returns rows in a different element order than Spark.

This is the ordering analog of the array_intersect ordering caveat already documented, and is separate from the NaN canonicalization divergence tracked in #4481.

Steps to reproduce

Compare array_union(a, b) element ordering between Comet and Spark for inputs where the union introduces new elements from the right array, and where the left array contains duplicates.

Expected behavior

Verify whether DataFusion's array_union preserves Spark's left-first-then-new-right-elements ordering. If it does, no change is needed and this can be closed. If it does not, raise CometArrayUnion's support level to Incompatible(Some(...)) with a documented reason, matching the array_intersect treatment.

Additional context

Split out from #4503 (item 5), surfaced by the audit-comet-expression skill run in #4483. Distinct from the closed array_union correctness issue #3644.