fix: return 422 for invalid historyLength in REST get_task

Replace unhandled int() conversion with Pydantic validation so
non-numeric historyLength query params (e.g. ?historyLength=abc)
return 422 InvalidParamsError instead of 500.

Author: azilbersteinSFDC

src/a2a/server/request_handlers/rest_handler.py

@@ -31,7 +31,7 @@
     SubscribeToTaskRequest,
 )
 from a2a.utils import proto_utils
-from a2a.utils.errors import ServerError, TaskNotFoundError
+from a2a.utils.errors import InvalidParamsError, ServerError, TaskNotFoundError
 from a2a.utils.helpers import validate
 from a2a.utils.telemetry import SpanKind, trace_class
 
@@ -249,7 +249,12 @@ async def on_get_task(
         """
         task_id = request.path_params['id']
         history_length_str = request.query_params.get('historyLength')
-        history_length = int(history_length_str) if history_length_str else None
+        try:
+            history_length = int(history_length_str) if history_length_str else None
+        except ValueError:
+            raise ServerError(
+                error=InvalidParamsError(message='historyLength must be a valid integer')
+            )
         params = GetTaskRequest(id=task_id, history_length=history_length)
         task = await self.request_handler.on_get_task(params, context)
         if task:

tests/server/apps/rest/test_rest_fastapi_app.py

@@ -396,5 +396,17 @@ async def test_send_message_rejected_task(
     assert expected_response == actual_response
 
 
+@pytest.mark.anyio
+async def test_get_task_invalid_history_length_returns_422(
+    client: AsyncClient,
+) -> None:
+    """Non-numeric historyLength query param returns 422 InvalidParamsError."""
+    response = await client.get('/v1/tasks/some-task-id?historyLength=abc')
+    assert response.status_code == 422
+    data = response.json()
+    assert 'message' in data
+    assert 'historylength' in data['message'].lower()
+
+
 if __name__ == '__main__':
     pytest.main([__file__])