Address PR review comments for FlinkArrowUtils

- Mark class as final to prevent subclassing
- Add null check for logicalType parameter to throw IllegalArgumentException instead of NPE

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: x-tong

auron-flink-extension/auron-flink-runtime/src/main/java/org/apache/auron/flink/arrow/FlinkArrowUtils.java

@@ -53,7 +53,7 @@
 /**
  * Utility class for converting Flink {@link LogicalType} instances to Arrow types, fields and schemas.
  */
-public class FlinkArrowUtils {
+public final class FlinkArrowUtils {
 
     /**
      * Root allocator for Arrow memory management.
@@ -82,6 +82,9 @@ public static BufferAllocator createChildAllocator(String name) {
      * @throws UnsupportedOperationException if the type is not supported
      */
     public static ArrowType toArrowType(LogicalType logicalType) {
+        if (logicalType == null) {
+            throw new IllegalArgumentException("logicalType cannot be null");
+        }
         if (logicalType instanceof NullType) {
             return ArrowType.Null.INSTANCE;
         } else if (logicalType instanceof BooleanType) {