test(planner): cover topN=0 and topN=-1 boundary in SuggestionsTest

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

backend/src/test/java/com/recipeapp/planning/SuggestionsTest.java

@@ -205,6 +205,28 @@ class SuggestionsTest {
                     .isInstanceOf(ResourceNotFoundException.class);
         }
 
+        @Test
+        void topNZeroShouldReturnEmptyList() {
+            var plan = createPlan();
+            stubPlan(plan);
+
+            SuggestionResponse result = planningService.getSuggestions(
+                    HOUSEHOLD_ID, plan.getId(), MONDAY, List.of(), 0);
+
+            assertThat(result.suggestions()).isEmpty();
+        }
+
+        @Test
+        void topNNegativeShouldReturnEmptyList() {
+            var plan = createPlan();
+            stubPlan(plan);
+
+            SuggestionResponse result = planningService.getSuggestions(
+                    HOUSEHOLD_ID, plan.getId(), MONDAY, List.of(), -1);
+
+            assertThat(result.suggestions()).isEmpty();
+        }
+
         @Test
         void singleCandidateShouldReturnOne() {
             var plan = createPlan();