Fix error handling for gRPC

Author: ishymko

src/a2a/compat/v0_3/grpc_handler.py

@@ -170,17 +170,17 @@ async def _handler(
             context, _handler, a2a_v0_3_pb2.SendMessageResponse()
         )
 
-    @validate_async_generator(
-        lambda self: self.agent_card.capabilities.streaming,
-        'Streaming is not supported by the agent',
-    )
     async def SendStreamingMessage(
         self,
         request: a2a_v0_3_pb2.SendMessageRequest,
         context: grpc.aio.ServicerContext,
     ) -> AsyncIterable[a2a_v0_3_pb2.StreamResponse]:
         """Handles the 'SendStreamingMessage' gRPC method (v0.3)."""
 
+        @validate_async_generator(
+            lambda _: self.agent_card.capabilities.streaming,
+            'Streaming is not supported by the agent',
+        )
         async def _handler(
             server_context: ServerCallContext,
         ) -> AsyncIterable[a2a_v0_3_pb2.StreamResponse]:
@@ -233,17 +233,17 @@ async def _handler(
 
         return await self._handle_unary(context, _handler, a2a_v0_3_pb2.Task())
 
-    @validate_async_generator(
-        lambda self: self.agent_card.capabilities.streaming,
-        'Streaming is not supported by the agent',
-    )
     async def TaskSubscription(
         self,
         request: a2a_v0_3_pb2.TaskSubscriptionRequest,
         context: grpc.aio.ServicerContext,
     ) -> AsyncIterable[a2a_v0_3_pb2.StreamResponse]:
         """Handles the 'TaskSubscription' gRPC method (v0.3)."""
 
+        @validate_async_generator(
+            lambda _: self.agent_card.capabilities.streaming,
+            'Streaming is not supported by the agent',
+        )
         async def _handler(
             server_context: ServerCallContext,
         ) -> AsyncIterable[a2a_v0_3_pb2.StreamResponse]:
@@ -260,17 +260,17 @@ async def _handler(
         async for item in self._handle_stream(context, _handler):
             yield item
 
-    @validate(
-        lambda self: self.agent_card.capabilities.push_notifications,
-        'Push notifications are not supported by the agent',
-    )
     async def CreateTaskPushNotificationConfig(
         self,
         request: a2a_v0_3_pb2.CreateTaskPushNotificationConfigRequest,
         context: grpc.aio.ServicerContext,
     ) -> a2a_v0_3_pb2.TaskPushNotificationConfig:
         """Handles the 'CreateTaskPushNotificationConfig' gRPC method (v0.3)."""
 
+        @validate(
+            lambda _: self.agent_card.capabilities.push_notifications,
+            'Push notifications are not supported by the agent',
+        )
         async def _handler(
             server_context: ServerCallContext,
         ) -> a2a_v0_3_pb2.TaskPushNotificationConfig: