Update tests/builders.py

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

Author: ahoblitz

tests/builders.py

@@ -200,12 +200,14 @@ def as_last_chunk(self) -> 'ArtifactUpdateEventBuilder':
         return self
 
     def build(self) -> TaskArtifactUpdateEvent:
-        if not self.artifact:
-            self.artifact = ArtifactBuilder().build()
+        artifact = self.artifact
+        if not artifact:
+            artifact = ArtifactBuilder().build()
         return TaskArtifactUpdateEvent(
             task_id=self.task_id,
             context_id=self.context_id,
-            artifact=self.artifact,
+            artifact=artifact,
             append=self.append,
             last_chunk=self.last_chunk,
         )
+