Return a copy of messages in MockMTAppender (elastic#94982)

Couldn't reproduce the failure, but in principle there is an issue
there. Other options are a CopyOnWriteArrayList or just synchronizing on
`messages()` return value. Returning a copy should be enough, since this
is only used in one place for asserting some test expectations in an
`assertBusy`.

Closes elastic#94559

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -2546,7 +2546,7 @@ private static class MockMTAppender extends AbstractAppender {
         private final List<String> messages = Collections.synchronizedList(new ArrayList<>());
 
         List<String> messages() {
-            return messages;
+            return List.copyOf(messages);
         }
 
         MockMTAppender(final String name) throws IllegalAccessException {