Add async validation support to FormTokenField

[Shekhar0109]

What?

Closes #71310

This PR adds async validation support to the FormTokenField component. It updates the token submission logic to handle both synchronous and asynchronous validation functions. When async validation fails, an error message is shown below the input.

Why?

Currently, FormTokenField only supports synchronous validation. This prevents integration with APIs or other asynchronous validation logic. Supporting async validation allows token input to be verified in real-time against external sources before being added.

How?

• Added isValidating and asyncError state to track validation state.
• Converted addNewToken to an async function that handles both sync and async validation.
• Display async validation error below the input using <StyledHelp> with a new .components-form-token-field__error CSS class.
• Updated SCSS to style error messages in red.
• Maintained existing synchronous validation fallback.

Testing Instructions

1. Open Gutenberg and add a block or form that uses FormTokenField.
2. Enter tokens that should pass and fail async validation.
3. Press Enter, Tab, or blur the input to trigger async validation.
4. Confirm that invalid tokens show the error message below the input.
5. Confirm valid tokens are added as expected.

Testing Instructions for Keyboard

• Navigate the tokens and suggestions using Tab, Shift+Tab, Enter, and arrow keys.
• Ensure that async validation errors are accessible via screen readers.

[github-actions]

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Co-authored-by: Shekhar0109 <shekh0109@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[github-actions]

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[t-hamano]

Thanks for the PR!

If I understand correctly, #71310 seems to target the components in the validated-form-controls directory.

cc @WordPress/gutenberg-components @oandregal

[Shekhar0109]

Thanks @t-hamano for clarifying! Yes, my intent was to address async validation in FormTokenField as part of the async refinements tracked in #71310. Please let me know if this should be scoped differently.

I noticed some failing checks:

I’ll update CHANGELOG.md for the components package.
I’ll run lint and unit tests locally to fix errors and add coverage for async validation.

I’ll push the fixes shortly 🚀

[mirka]

Validation support is being implemented in a more centralized way, and support for FormTokenField specifically is being worked on in #71350.

[Mamaduka]

Thanks for contributing, @Shekhar0109!

It appears that the same task is already in progress, so I think we can close the PR.

I think you're a first-time contributor? You're welcome to contribute in whatever way you want, but general advice is to start with the 'Good First Issue' label that is usually more suited for first contributors - https://github.com/WordPress/gutenberg/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Good%20First%20Issue%22.

Don't forget to check the contribution guidelines - https://developer.wordpress.org/block-editor/contributors/code/. Configuring your development environment and following guidelines is important for an open-source project.