fix(spaces): Ensure space children ordering is transitive

This patch fixes the SpaceRoom::compare_rooms method to be transitive
(If A ≤ B and B ≤ C then A ≤ C). The transitive property of a comparison
function is required by the sorting functions we are using.

If the property doesn't hold, sorting will panic.

The previous logic violated strict weak ordering by falling back towards
room ID comparison as soon as one of the values doesn't have a
SpaceRoomChildState.

This introduced inconsistent ordering paths where:
    - A and B would be compared using the SpaceRoomChildState
    - B and C would be compared using only the room ID
    - A and C would be compared using only the room ID

Leading to the case where:
    - A < B due to the state
    - B < C due to the room ID
    - and finally A > C due to the room ID.

As a result, transitivity could be broken (A < B, B < C, but C < A),
leading to a panic during the sorting.

crates/matrix-sdk-ui/src/spaces/room.rs

@@ -168,7 +168,9 @@ impl SpaceRoom {
                     .cmp(&b_state.origin_server_ts)
                     .then(a_room_id.cmp(b_room_id)),
             },
-            _ => a_room_id.cmp(b_room_id),
+            (None, Some(_)) => Ordering::Greater,
+            (Some(_), None) => Ordering::Less,
+            (None, None) => a_room_id.cmp(b_room_id),
         }
     }
 }