fix: merge metadata appendArtifactToTask metadata

Author: mhyeon-lee

server-common/src/test/java/io/a2a/server/tasks/TaskManagerTest.java

@@ -190,6 +190,7 @@ public void testTaskArtifactUpdateEventAppendTrueWithExistingArtifact() throws A
                 .artifactId("artifact-id")
                 .name("artifact-1")
                 .parts(Collections.singletonList(new TextPart("existing content")))
+                .metadata(Map.of("key1", "value1"))
                 .build();
         Task taskWithArtifact = Task.builder(initialTask)
                 .artifacts(Collections.singletonList(existingArtifact))
@@ -201,6 +202,7 @@ public void testTaskArtifactUpdateEventAppendTrueWithExistingArtifact() throws A
                 .artifactId("artifact-id")
                 .name("artifact-1")
                 .parts(Collections.singletonList(new TextPart("new content")))
+                .metadata(Map.of("key2", "value2"))
                 .build();
         TaskArtifactUpdateEvent event = TaskArtifactUpdateEvent.builder()
                 .taskId(minimalTask.id())
@@ -217,6 +219,9 @@ public void testTaskArtifactUpdateEventAppendTrueWithExistingArtifact() throws A
         assertEquals(2, updatedArtifact.parts().size());
         assertEquals("existing content", ((TextPart) updatedArtifact.parts().get(0)).text());
         assertEquals("new content", ((TextPart) updatedArtifact.parts().get(1)).text());
+
+        assertEquals("value1", updatedArtifact.metadata().get("key1"));
+        assertEquals("value2", updatedArtifact.metadata().get("key2"));
     }
 
     @Test

spec/src/main/java/io/a2a/util/Utils.java

@@ -3,7 +3,9 @@
 import static io.a2a.util.Assert.checkNotNullParam;
 
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.logging.Logger;
 
 import com.google.gson.Gson;
@@ -129,8 +131,18 @@ public static Task appendArtifactToTask(Task task, TaskArtifactUpdateEvent event
             log.fine(String.format("Appending parts to artifact id %s for task %s", artifactId, taskId));
             List<Part<?>> parts = new ArrayList<>(existingArtifact.parts());
             parts.addAll(newArtifact.parts());
+
+            Map<String, Object> metadata = new HashMap<>();
+            if (existingArtifact.metadata() != null) {
+                metadata.putAll(existingArtifact.metadata());
+            }
+            if (newArtifact.metadata() != null) {
+                metadata.putAll(newArtifact.metadata());
+            }
+
             Artifact updated = Artifact.builder(existingArtifact)
                     .parts(parts)
+                    .metadata(metadata)
                     .build();
             artifacts.set(existingArtifactIndex, updated);
         } else {