map_from_arrays falls back to Spark under mapKeyDedupPolicy=LAST_WIN, unlike map_from_entries #5589

Part of #5572.

map_from_arrays reports Incompatible whenever spark.sql.mapKeyDedupPolicy is LAST_WIN:

if (MapKeyDedupPolicySupport.isLastWin) {
  Incompatible(Some(MapKeyDedupPolicySupport.incompatibleReason))
}

spark/src/main/scala/org/apache/comet/serde/maps.scala:163-164

With no CodegenDispatchFallback mixin, that means the whole projection falls back to Spark for any session running that policy.

CometMapFromEntries sits directly below it in the same file and already carries the mixin (:211-213) for its own analogous restriction, so the two closely related functions behave differently for no good reason.

MapFromArrays.doGenCode builds the map through ArrayBasedMapBuilder, which is where the dedup policy is actually implemented, so the dispatcher gets LAST_WIN right by construction. MapType is supported by CometBatchKernelCodegen.isSupportedDataType recursively.

Fix: mix CodegenDispatchFallback into CometMapFromArrays.

Not in scope: the separate null-key divergence documented in getCompatibleNotes and tracked as #4680. That one is a Compatible path today, so it is a correctness question rather than a fallback question — though it is worth noting that routing through the dispatcher would fix it too, since Spark's builder raises "Cannot use null as map key" itself.