Update src/a2a/utils/artifact.py

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: kevinlu310

src/a2a/utils/artifact.py

@@ -139,15 +139,15 @@ def append(self, text: str) -> TaskArtifactUpdateEvent:
             ``last_chunk=False``.
         """
         return TaskArtifactUpdateEvent(
-            context_id=self.context_id,
-            task_id=self.task_id,
+            context_id=self._context_id,
+            task_id=self._task_id,
             append=True,
             last_chunk=False,
             artifact=Artifact(
-                artifact_id=self.artifact_id,
-                name=self.name,
+                artifact_id=self._artifact_id,
+                name=self._name,
                 parts=[Part(root=TextPart(text=text))],
-            ),
+            )
         )
 
     def finalize(self) -> TaskArtifactUpdateEvent: