fix: gemini

Signed-off-by: Shingo OKAWA <shingo.okawa.g.h.c@gmail.com>

Author: ognis1205

tests/client/test_grpc_client.py

@@ -3,8 +3,6 @@
 import grpc
 import pytest
 
-from google.protobuf import struct_pb2
-
 from a2a.client import A2AGrpcClient
 from a2a.grpc import a2a_pb2, a2a_pb2_grpc
 from a2a.types import (
@@ -107,7 +105,7 @@ def sample_artifact() -> Artifact:
         name='example.txt',
         description='An example artifact',
         parts=[Part(root=TextPart(text='Hi there'))],
-        metadata=struct_pb2.Struct(),
+        metadata={},
         extensions=[],
     )
 
@@ -120,7 +118,7 @@ def sample_task_status_update_event() -> TaskStatusUpdateEvent:
         contextId='ctx-1',
         status=TaskStatus(state=TaskState.working),
         final=False,
-        metadata=struct_pb2.Struct(),
+        metadata={},
     )
 
 
@@ -135,7 +133,7 @@ def sample_task_artifact_update_event(
         artifact=sample_artifact,
         append=True,
         last_chunk=True,
-        metadata=struct_pb2.Struct(),
+        metadata={},
     )