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

Part of #5572.

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

```scala
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.