fix: remove error types from enqueable events (#138)

Author: kthota-g

src/a2a/server/events/event_queue.py

@@ -3,8 +3,6 @@
 import sys
 
 from a2a.types import (
-    A2AError,
-    JSONRPCError,
     Message,
     Task,
     TaskArtifactUpdateEvent,
@@ -21,8 +19,6 @@
     | Task
     | TaskStatusUpdateEvent
     | TaskArtifactUpdateEvent
-    | A2AError
-    | JSONRPCError
 )
 """Type alias for events that can be enqueued."""