fix(combinations): guard null response; reject self-combination at load

- Combinations.tryUse: only write response when non-null and non-blank,
  preventing NPE/IllegalArgument for effects-only recipes.
- CombinationFactory.fromDto: throw WorldLoadException when a == b,
  since item singletons make a self-combination impossible at runtime.
- Tests: +effectsOnlyRecipeAppliesEffectsWithoutProduceOrResponse
  (CombinationsTest 7→8), +rejectsSelfCombination
  (CombinationFactoryTest 4→5).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Semesterprojekt/src/main/java/thb/jeanluc/adventure/game/Combinations.java

@@ -47,7 +47,9 @@ public final class Combinations {
             }
         }
         Effects.applyAll(c.effects(), ctx);
-        ctx.getIo().write(c.response());
+        if (c.response() != null && !c.response().isBlank()) {
+            ctx.getIo().write(c.response());
+        }
     }
 
     private static boolean present(GameContext ctx, String id) {

Semesterprojekt/src/main/java/thb/jeanluc/adventure/loader/CombinationFactory.java

@@ -18,6 +18,10 @@ public final class CombinationFactory {
     public static Combination fromDto(CombinationDto dto, Map<String, Item> items) {
         requireItem(items, dto.a(), "a");
         requireItem(items, dto.b(), "b");
+        if (dto.a().equals(dto.b())) {
+            throw new WorldLoadException(
+                    "Combination cannot combine an item with itself: '" + dto.a() + "'");
+        }
         if (dto.consume() != null) {
             for (String id : dto.consume()) {
                 requireItem(items, id, "consume");