test(form-core): cover onMount field errors before field mount - #2223

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: autofix-ci/action@7a166d7532b277f34e16238930461bf77f9d7ed8. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/