Stop returning `unsigned.invite_room_state` in `PUT /_matrix/federation/v2/invite/{roomId}/{eventId}` responses (#14064)

Co-authored-by: David Robertson <davidr@element.io>

changelog.d/14064.bugfix

@@ -0,0 +1 @@
+ Fix a long-standing bug where Synapse would accidentally include extra information in the response to [`PUT /_matrix/federation/v2/invite/{roomId}/{eventId}`](https://spec.matrix.org/v1.4/server-server-api/#put_matrixfederationv2inviteroomideventid).

synapse/federation/transport/server/federation.py

@@ -499,6 +499,11 @@ class FederationV2InviteServlet(BaseFederationServerServlet):
         result = await self.handler.on_invite_request(
             origin, event, room_version_id=room_version
         )
+
+        # We only store invite_room_state for internal use, so remove it before
+        # returning the event to the remote homeserver.
+        result["event"].get("unsigned", {}).pop("invite_room_state", None)
+
         return 200, result