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

@@ -47,8 +47,10 @@ public final class Combinations {
             }
         }
         Effects.applyAll(c.effects(), ctx);
+        if (c.response() != null && !c.response().isBlank()) {
             ctx.getIo().write(c.response());
         }
+    }
 
     private static boolean present(GameContext ctx, String id) {
         return ctx.getPlayer().hasItem(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");

Semesterprojekt/src/test/java/thb/jeanluc/adventure/game/CombinationsTest.java

@@ -123,6 +123,20 @@ class CombinationsTest {
         assertThat(ctx.getPlayer().hasItem("matches")).isTrue(); // not consumed
     }
 
+    @Test
+    void effectsOnlyRecipeAppliesEffectsWithoutProduceOrResponse() {
+        Combination effectsOnly = new Combination("matches", "shovel",
+                java.util.List.of(), java.util.List.of(), null,
+                java.util.List.of(new Effect(Effect.Type.SET_FLAG, "rite_done")),
+                null, null);
+        GameContext ctx = ctx(effectsOnly);
+        give(ctx, "matches");
+        give(ctx, "shovel");
+        Combinations.tryUse(ctx, "matches", "shovel");
+        assertThat(ctx.getState().isSet("rite_done")).isTrue();
+        assertThat(out(ctx)).doesNotContain("null");
+    }
+
     @Test
     void metRequiresAppliesEffectsAndProduces() {
         Combination gated = new Combination("matches", "torch",

Semesterprojekt/src/test/java/thb/jeanluc/adventure/loader/CombinationFactoryTest.java

@@ -58,4 +58,12 @@ class CombinationFactoryTest {
                 .isInstanceOf(WorldLoadException.class)
                 .hasMessageContaining("wraith");
     }
+
+    @Test
+    void rejectsSelfCombination() {
+        CombinationDto dto = new CombinationDto("torch", "torch", null, null, null, null, "x", null);
+        assertThatThrownBy(() -> CombinationFactory.fromDto(dto, items()))
+                .isInstanceOf(WorldLoadException.class)
+                .hasMessageContaining("itself");
+    }
 }