fix: handle nil/missing deactivated_at field in UpdateProjectState filter

The DeactivatedAt field uses omitempty tag, so it may not exist in MongoDB
for members who were never deactivated. Changed the filter to use
$not $gt pattern which correctly matches:
- deactivated_at = 0
- deactivated_at = nil
- field doesn't exist

Addresses Copilot code review feedback.

Author: hxrshxz

chaoscenter/authentication/pkg/project/repository.go

@@ -373,7 +373,7 @@ func (r repository) UpdateProjectState(ctx context.Context, userID string, deact
 				{"$elemMatch", bson.D{
 					{"user_id", bson.D{{"$ne", userID}}},
 					{"role", entities.RoleOwner},
-					{"deactivated_at", 0},
+					{"deactivated_at", bson.D{{"$not", bson.D{{"$gt", int64(0)}}}}},
 				}},
 			}},
 		}},