Made DurationArgument a final class

Ensured immutability by declaring `DurationArgument` as final, preventing unintended subclassing.

Author: NonSwag

paper/src/main/java/core/paper/command/argument/DurationArgument.java

@@ -13,7 +13,7 @@
  * A custom argument type implementation to handle durations in a converted format.
  */
 @NullMarked
-public class DurationArgument implements CustomArgumentType.Converted<Duration, Integer> {
+public final class DurationArgument implements CustomArgumentType.Converted<Duration, Integer> {
     private final Duration minimum;
 
     private DurationArgument(Duration minimum) {