checker: add domain length validation and refactor rules into per-concern checks

checker/interactive.go

@@ -45,6 +45,9 @@ func (p *xmppProvider) ParseForm(r *http.Request) (sdk.CheckerOptions, error) {
 	if domain == "" {
 		return nil, errors.New("domain is required")
 	}
+	if err := validateDomain(domain); err != nil {
+		return nil, err
+	}
 
 	opts := sdk.CheckerOptions{"domain": domain}