FINERACT-2534: Add unit tests for StreamUtil utility class

Add unit tests for:
- foldLeft collector functionality
- mergeMapsOfLists collector functionality

Include edge case tests for empty streams and duplicate map keys.

Improves reliability and test coverage for StreamUtil.

Author: devi-pathak2263

fineract-core/src/test/java/org/apache/fineract/util/StreamUtilTest.java

@@ -0,0 +1,73 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.fineract.util;
+
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class StreamUtilTest {
+
+    @Test
+    void foldLeft() {
+        List<Integer> numbers = List.of(1, 2, 3, 4);
+
+        Integer result = numbers.stream().collect(StreamUtil.foldLeft(0, Integer::sum));
+
+        Assertions.assertEquals(10, result);
+    }
+
+    @Test
+    void foldLeftWithEmptyStream() {
+        List<Integer> numbers = List.of();
+
+        Integer result = numbers.stream().collect(StreamUtil.foldLeft(0, Integer::sum));
+
+        Assertions.assertEquals(0, result);
+    }
+
+    @Test
+    void mergeMapsOfLists() {
+        Map<String, List<Integer>> map1 = Map.of("a", List.of(1, 2));
+        Map<String, List<Integer>> map2 = Map.of("a", List.of(3, 4));
+
+        Map<String, List<Integer>> merged = Stream.of(map1, map2).collect(StreamUtil.mergeMapsOfLists());
+
+        Assertions.assertEquals(List.of(1, 2, 3, 4), merged.get("a"));
+    }
+
+    @Test
+    void mergeMapsOfListsWithEmptyStream() {
+        Map<String, List<Integer>> merged = Stream.<Map<String, List<Integer>>>of().collect(StreamUtil.mergeMapsOfLists());
+
+        Assertions.assertTrue(merged.isEmpty());
+    }
+
+    @Test
+    void mergeMapsOfListsWhenSameKeyAppearsInMultipleMaps() {
+        Map<String, List<Integer>> map1 = Map.of("a", List.of(1, 2));
+        Map<String, List<Integer>> map2 = Map.of("a", List.of(3, 4));
+
+        Map<String, List<Integer>> merged = Stream.of(map1, map2).collect(StreamUtil.mergeMapsOfLists());
+
+        Assertions.assertEquals(List.of(1, 2, 3, 4), merged.get("a"));
+    }
+}