Default to public join rule in remote summary (#18493)

See: https://github.com/element-hq/synapse/issues/18358#issuecomment-2866119550

changelog.d/18493.bugfix

@@ -0,0 +1 @@
+Fixed room summary API incorrectly returning that a room is private in the room summary response when the join rule is omitted by the remote server. Contributed by @nexy7574.

synapse/handlers/room_summary.py

@@ -701,7 +701,7 @@ class RoomSummaryHandler:
         # The API doesn't return the room version so assume that a
         # join rule of knock is valid.
         if (
-            room.get("join_rule")
+            room.get("join_rule", JoinRules.PUBLIC)
             in (JoinRules.PUBLIC, JoinRules.KNOCK, JoinRules.KNOCK_RESTRICTED)
             or room.get("world_readable") is True
         ):