Since Python 3.11 and [bpo-40066](python/cpython#84247), Enum.__str__ shows the enum name.

Adjust the error message to only show the string value.

Author: amauryfa

src/a2a/server/request_handlers/default_request_handler.py

@@ -195,7 +195,7 @@ async def _setup_message_execution(
             if task.status.state in TERMINAL_TASK_STATES:
                 raise ServerError(
                     error=InvalidParamsError(
-                        message=f'Task {task.id} is in terminal state: {task.status.state}'
+                        message=f'Task {task.id} is in terminal state: {task.status.state.value}'
                     )
                 )
 
@@ -437,7 +437,7 @@ async def on_resubscribe_to_task(
         if task.status.state in TERMINAL_TASK_STATES:
             raise ServerError(
                 error=InvalidParamsError(
-                    message=f'Task {task.id} is in terminal state: {task.status.state}'
+                    message=f'Task {task.id} is in terminal state: {task.status.state.value}'
                 )
             )