Use the proper serialization format when bundling aggregations. (#12090)

This ensures that the `latest_event` field of the bundled aggregation
for threads uses the same format as the other events in the response.

synapse/handlers/events.py

@@ -19,6 +19,7 @@ from typing import TYPE_CHECKING, Iterable, List, Optional
 from synapse.api.constants import EduTypes, EventTypes, Membership
 from synapse.api.errors import AuthError, SynapseError
 from synapse.events import EventBase
+from synapse.events.utils import SerializeEventConfig
 from synapse.handlers.presence import format_user_presence_state
 from synapse.streams.config import PaginationConfig
 from synapse.types import JsonDict, UserID
@@ -120,7 +121,7 @@ class EventStreamHandler:
             chunks = self._event_serializer.serialize_events(
                 events,
                 time_now,
-                as_client_event=as_client_event,
+                config=SerializeEventConfig(as_client_event=as_client_event),
             )
 
             chunk = {