DataForm: better story for validation

[oandregal]

What?

Improves the validation story for DataForm, including custom logic and using the panel layout.

Why?

To better describe all the states we need to cover.

Testing Instructions

Visit the storybook (npm install && npm run storybook:dev) and verify the changes.

[github-actions]

Flaky tests detected in d5d9d6d.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/17148425154
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js

[github-actions]

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Co-authored-by: oandregal <[email protected]>

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