Reject INCREMENT/SET_STEPS_AT_LEAST without steps

Without this, a malformed payload would NPE on auto-unboxing inside the
switch and produce a confusing stack trace. Throwing explicitly points
straight at the contract violation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Brutus5000

src/main/java/com/faforever/api/achievements/AchievementsController.java

@@ -21,9 +21,16 @@ public void update(AchievementUpdateRequest request) {
     switch (request.operation()) {
       case REVEAL -> throw new UnsupportedOperationException("REVEAL is not yet implemented");
       case UNLOCK -> achievementService.unlock(request.playerId(), request.achievementId());
-      case INCREMENT -> achievementService.increment(request.playerId(), request.achievementId(), request.steps());
+      case INCREMENT -> achievementService.increment(request.playerId(), request.achievementId(), requireSteps(request));
       case SET_STEPS_AT_LEAST ->
-        achievementService.setStepsAtLeast(request.playerId(), request.achievementId(), request.steps());
+        achievementService.setStepsAtLeast(request.playerId(), request.achievementId(), requireSteps(request));
     }
   }
+
+  private static int requireSteps(AchievementUpdateRequest request) {
+    if (request.steps() == null) {
+      throw new IllegalArgumentException("steps is required for operation " + request.operation());
+    }
+    return request.steps();
+  }
 }