feat: release managed service generation and contract diagnostics

config/config.go

@@ -56,14 +56,18 @@ func (c Config) Validate() error {
 	if err := c.Errors.Validate(); err != nil {
 		return err
 	}
-	if c.ResponseEncoder == nil {
+	encoder := c.ResponseEncoder
+	if encoder == nil {
 		f, err := response.New(c.Response)
 		if err != nil {
 			return err
 		}
-		if err = f.ValidateErrors(c.Errors); err != nil {
-			return err
-		}
+		encoder = f
+	}
+	// Check the same composition that App.New installs, including custom
+	// encoders and their existing catalogs, before App construction.
+	if _, err := response.WithErrors(c.Errors, encoder); err != nil {
+		return err
 	}
 	if err := c.Response.Validate(); err != nil {
 		return err