feat: allow players to leave sessions

src/GmRelay.Shared/Domain/SessionCapacityRules.cs

@@ -23,4 +23,14 @@ public static class SessionCapacityRules
 
         return !maxPlayers.HasValue || activeParticipants < maxPlayers.Value;
     }
+
+    public static bool ShouldPromoteAfterParticipantLeaves(
+        string removedRegistrationStatus,
+        int? maxPlayers,
+        int activeParticipantsAfterLeave,
+        int waitlistedParticipants)
+    {
+        return removedRegistrationStatus == ParticipantRegistrationStatus.Active
+            && CanPromoteWaitlistedPlayer(maxPlayers, activeParticipantsAfterLeave, waitlistedParticipants);
+    }
 }