fix: double check the constraint error if save failed

Author: borosr

openmeter/subscription/addon/repo/subscriptionaddon.go

@@ -2,8 +2,10 @@ package subscriptionaddonrepo
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/openmeterio/openmeter/openmeter/ent/db"
 	dbsubscriptionaddon "github.com/openmeterio/openmeter/openmeter/ent/db/subscriptionaddon"
 	dbsubscriptionaddonquantity "github.com/openmeterio/openmeter/openmeter/ent/db/subscriptionaddonquantity"
@@ -45,9 +47,12 @@ func (r *subscriptionAddonRepo) Create(ctx context.Context, namespace string, in
 			// Keep the user-visible message generic so it does not echo addon/subscription IDs;
 			// the workflow's pre-Create validation already establishes that both exist.
 			if db.IsConstraintError(err) {
-				return nil, models.NewGenericConflictError(
-					fmt.Errorf("addon is already attached to subscription"),
-				)
+				var pgErr *pgconn.PgError
+				if errors.As(err, &pgErr) && pgErr.Code == "23505" {
+					return nil, models.NewGenericConflictError(
+						fmt.Errorf("addon is already attached to subscription"),
+					)
+				}
 			}
 
 			return nil, err