fix: invalid label NoInteractive exit code (#683)

* add test
* return validation error not nil from function

Closes #665

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/683
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.com>
Co-authored-by: Christopher Homberger <christopher.homberger@web.de>
Co-committed-by: Christopher Homberger <christopher.homberger@web.de>

internal/app/cmd/register_test.go

@@ -0,0 +1,19 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestRegisterNonInteractiveReturnsLabelValidationError(t *testing.T) {
+	err := registerNoInteractive(t.Context(), "", &registerArgs{
+		Labels:       "label:invalid",
+		Token:        "token",
+		InstanceAddr: "http://localhost:3000",
+	})
+	assert.Error(t, err, "unsupported schema: invalid")
+}