DataForm: Streamline validation behavior

[youknowriad]

What?

This PR proposes a better Dev Experience when working with DataForm and validation. Instead of assuming the consumer is going to run 'isltemValid' manually and think about when and how to run validation properly, this PR runs validation on each change and passes the result as a second argument for the onChange callback.
This is kind of inline with how most form libraries work, they take care of the validation... and they only give you a state.

Testing Instructions

There shouldn't be any impact, the only place that uses validation right now is the "reoder page" action. You can test that action quickly or the validation story in storybook.

[github-actions]

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Co-authored-by: youknowriad <youknowriad@git.wordpress.org>
Co-authored-by: oandregal <oandregal@git.wordpress.org>

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[github-actions]

Flaky tests detected in 8b66d61.
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/17231116306
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js
• [Flaky Test] ensures List View global shortcut works properly #51533 in /test/e2e/specs/editor/various/list-view.spec.js

[oandregal]

I'd like to leave this export for now: it can be useful in non-reactive situations (e.g., to validate the form data on load).

[youknowriad]

Ok works for me.

[oandregal]

what's this?

[youknowriad]

An idea that I decided to ditch :P

[oandregal]

@youknowriad while working on adding support for async validation, I just realized this PR is problematic:

• We need to handle async validation, but we don't want to re-trigger onChange when we get the results of the validation (only when the content changes). I think we need to introduce a separate onValidate callback instead.
• This is disconnected from the components, causing issues. For example, the 1st time, components do not trigger validation onChange, but onBlur. So, by doing this, we run into the following: the onChange callback will say a form is invalid while the component hasn't yet. By exposing an onValidate callback that is triggered by the leaf component onValidate we fix this as well.
• This triggers validation twice: first, when the component validates; second, when the isItemValid is executed. With the goal of minimizing performance impact (e.g., triggering potentially time-consuming custom validations), we should look at triggering only once.

I'm working on addressing those in #71161. I'd like to revert this PR and focus on landing 71161 instead.