GH-8779: MongoDbMS Fix distinct result conversion (#8781)

Fixes #8779

The `MongoTemplate` has now a `findDistinct()` API with a smart
result conversion instead of native driver plain expectations

**Cherry-pick to `6.1.x` & `6.0.x`**

Author: artembilan

spring-integration-mongodb/src/main/java/org/springframework/integration/mongodb/store/ConfigurableMongoDbMessageStore.java

@@ -239,8 +239,9 @@ public void completeGroup(Object groupId) {
 	@Override
 	public Iterator<MessageGroup> iterator() {
 		Query query = Query.query(Criteria.where(MessageDocumentFields.GROUP_ID).exists(true));
-		Iterable<String> groupIds = getMongoTemplate().getCollection(collectionName)
-				.distinct(MessageDocumentFields.GROUP_ID, query.getQueryObject(), String.class);
+		Iterable<Object> groupIds =
+				getMongoTemplate()
+						.findDistinct(query, MessageDocumentFields.GROUP_ID, this.collectionName, Object.class);
 
 		return StreamSupport.stream(groupIds.spliterator(), false)
 				.map(this::getMessageGroup)

spring-integration-mongodb/src/main/java/org/springframework/integration/mongodb/store/MongoDbMessageStore.java

@@ -367,8 +367,7 @@ public Iterator<MessageGroup> iterator() {
 
 		Query query = Query.query(Criteria.where(GROUP_ID_KEY).exists(true));
 
-		Iterable<String> groupIds = this.template.getCollection(this.collectionName)
-				.distinct(GROUP_ID_KEY, query.getQueryObject(), String.class);
+		Iterable<Object> groupIds = this.template.findDistinct(query, GROUP_ID_KEY, this.collectionName, Object.class);
 
 		for (Object groupId : groupIds) {
 			messageGroups.add(getMessageGroup(groupId));

spring-integration-mongodb/src/test/java/org/springframework/integration/mongodb/store/AbstractMongoDbMessageGroupStoreTests.java

@@ -374,7 +374,7 @@ void testMessageGroupIterator() {
 		Message<?> message = new GenericMessage<>("1");
 		store2.addMessagesToGroup("1", message);
 		store1.addMessagesToGroup("2", new GenericMessage<>("2"));
-		store2.addMessagesToGroup("3", new GenericMessage<>("3"));
+		store2.addMessagesToGroup(UUID.randomUUID(), new GenericMessage<>("3"));
 
 		MessageGroupStore store3 = this.getMessageGroupStore();
 		Iterator<MessageGroup> iterator = store3.iterator();