fix(message): preserve response topic when copying

src/main/java/io/mapsmessaging/api/MessageBuilder.java

@@ -86,6 +86,7 @@ public MessageBuilder(Message previousMessage) {
     expiry = (previousMessage.getExpiry() - System.currentTimeMillis());
     creation = previousMessage.getCreation();
     contentType = previousMessage.getContentType();
+    responseTopic = previousMessage.getResponseTopic();
     if(previousMessage.getCorrelationData() != null && !previousMessage.isCorrelationDataByteArray()) {
       correlationData = new String(previousMessage.getCorrelationData());
     }

src/test/java/io/mapsmessaging/api/MessageBuilderTest.java

@@ -137,6 +137,7 @@ void copyConstructor_preservesMessageFields_andIsolatesMaps() {
     Message original = new MessageBuilder()
         .setCreation(creation)
         .setSchemaId("schema-id")
+        .setResponseTopic("reply/topic")
         .setMeta(meta)
         .setDataMap(dataMap)
         .build();
@@ -145,6 +146,7 @@ void copyConstructor_preservesMessageFields_andIsolatesMaps() {
 
     Assertions.assertEquals(creation, copy.getCreation());
     Assertions.assertEquals("schema-id", copy.getSchemaId());
+    Assertions.assertEquals("reply/topic", copy.getResponseTopic());
     Assertions.assertNotSame(original.getMeta(), copy.getMeta());
     Assertions.assertNotSame(original.getDataMap(), copy.getDataMap());
 
@@ -153,4 +155,13 @@ void copyConstructor_preservesMessageFields_andIsolatesMaps() {
     Assertions.assertFalse(original.getMeta().containsKey("copy-meta"));
     Assertions.assertFalse(original.getDataMap().containsKey("copy-data"));
   }
+
+  @Test
+  void copyConstructor_preservesNullResponseTopic() {
+    Message original = new MessageBuilder().build();
+
+    Message copy = new MessageBuilder(original).build();
+
+    Assertions.assertNull(copy.getResponseTopic());
+  }
 }

src/test/java/io/mapsmessaging/engine/destination/MessageOverridesTest.java

@@ -138,6 +138,7 @@ void applyOverrides_mergesMeta_andDataMap_creatingMapsWhenNull() {
 
     Assertions.assertEquals("x", updated.getDataMap().get("d1").getData());
     Assertions.assertEquals(42L, updated.getDataMap().get("d2").getData());
+    Assertions.assertEquals("resp/topic", updated.getResponseTopic());
     Assertions.assertFalse(message.getMeta().containsKey("m2"));
     Assertions.assertFalse(message.getDataMap().containsKey("d2"));
   }