feat(form-core): add validateArrayFields for validating nested fields

[vojty]

🎯 Changes

This PR introduces a new function validateArrayFields that is similar to validateArrayFieldsStartingFrom but it validates only nested fields for the given index.

✅ Checklist

• [ ] I have followed the steps in the Contributing guide.
• [x] I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• [ ] This change affects published code, and I have generated a changeset.
• [ ] This change is docs/CI/dev-only (no release).

[changeset-bot[bot]]

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Latest commit: 8c7fa1de48e488615c5155cc2a46497cee1a8586

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