Fix event context for outliers in important MSC2716 spot (#10938)

Fix event context for outlier causing failures in all of the MSC2716
Complement tests.

The `EventContext.for_outlier` refactor happened in
#10883
and this spot was left out.

changelog.d/10938.bugfix

@@ -0,0 +1 @@
+Fix bug introduced in Synapse 1.44 which caused the experimental [MSC2716](https:/matrix-org/matrix-doc/pull/2716) `/batch_send` endpoint to return a 500 error.

synapse/handlers/message.py

@@ -952,18 +952,13 @@ async def create_new_client_event(
  depth=depth,
  )
 
- old_state = None
-
  # Pass on the outlier property from the builder to the event
  # after it is created
  if builder.internal_metadata.outlier:
- event.internal_metadata.outlier = builder.internal_metadata.outlier
-
- # Calculate the state for outliers that pass in their own `auth_event_ids`
- if auth_event_ids:
- old_state = await self.store.get_events_as_list(auth_event_ids)
-
- context = await self.state.compute_event_context(event, old_state=old_state)
+ event.internal_metadata.outlier = True
+ context = EventContext.for_outlier()
+ else:
+ context = await self.state.compute_event_context(event)
 
  if requester:
  context.app_service = requester.app_service