fix: replace err.Error() string match with errors.Is(err, pgx.ErrTxClosed)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/payments/service.go

@@ -2,8 +2,8 @@ package payments
 
 import (
 	"context"
-	"crussell/db"
 	"crussell/clock"
+	"crussell/db"
 	"crussell/internal/square"
 	"errors"
 	"fmt"
@@ -470,10 +470,10 @@ func (s *PaymentService) DeletePaymentMethod(ctx context.Context, cardID, userID
 		return err
 	}
 	defer func() {
-    if err := tx.Rollback(ctx); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	retainedUntil := clock.Now().Add(7 * 365 * 24 * time.Hour)
 	_, err = tx.Exec(ctx, `