fix: provide fallback reason for ArrayFilter operator

CometArrayFilter.getSupportLevel returned Unsupported() with no reason
for unsupported filter functions, resulting in empty fallback messages.
Add an explicit reason string.

Author: andygrove

spark/src/main/scala/org/apache/comet/serde/arrays.scala

@@ -622,7 +622,8 @@ object CometArrayFilter extends CometExpressionSerde[ArrayFilter] {
   override def getSupportLevel(expr: ArrayFilter): SupportLevel = {
     expr.function.children.headOption match {
       case Some(_: IsNotNull) => Compatible()
-      case _ => Unsupported()
+      case _ =>
+        Unsupported(Some("Only array_compact (ArrayFilter with IsNotNull) is supported"))
     }
   }