Fix checkbox and radio input validation positioning

[MelissaPastore]

Summary

Ref: #203

This PR fixes a bug with the positioning of the validation message for checkbox and radio inputs inside a FormControl component. This was originally reported as just an issue for checkboxes, but I also noticed it impacted radio inputs as well, so this PR fixes the issue for both types of inputs.

List of notable changes:

• updated the .FormControl--checkbox css class to use flex-grid styling instead of flexbox with row-reverse
• added FormControl-validation-checkbox css class to position the validation message for checkboxes and radio buttons.
• added the FormControl-validation-checkbox class to any FormControlValidation children only when there is a checkbox or radio input.
• added a storybook story for validation with checkbox and radio inputs to illustrate what these scenarios look like.
• updated the CSS classes for the wrappers for radio and checkbox inputs to conditionally use the grid-row property when their parent element is a form control element.

What should reviewers focus on?

• What do you think of this approach of using CSS to fix this issue? I saw there was a previous PR that made an attempt to modify the FormControl component's composition and place a container around the input and its label so the validation message would no longer share a parent with them and therefore not be affected by the row-reverse property on the parent container. This CSS only approach seemed a little simpler but I'm not sure if it's good practice to have a position: absolute element within a flex container?

Steps to test:

1. Open the FormControl component in CI-deployed preview environment
2. Go to 'w/ Error Validation with Checkbox - Playground' and 'w/ Error Validation with Radio - Playground' story in Storybook
3. Verify that the validation now appears below the checkbox and radio inputs instead of next to them.

Supporting resources (related issues, external links, etc):

• Related issue: 🐛 [BUG - React] - FormControl.Validation layout problems with Checkbox #203
• Previous PR that attempted to fix this issue: Attempt to fix validation around checkbox #513

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• New visual snapshots have been generated / updated for any UI changes
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Please try to provide before and after screenshots or videos

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: 6fcda52

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@primer/react-brand	Patch
@primer/brand-primitives	Patch
@primer/brand-e2e	Patch
@primer/brand-fonts	Patch
@primer/brand-config	Patch
@primer/brand-storybook	Patch

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[github-actions]

🟢 No design token changes found

[github-actions]

⚠️ Visual differences found

Our visual comparison tests found UI differences.

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[MelissaPastore]

This is pretty much borrowed from the CheckboxPlayground implementation so I could re-use this component for the example with a validation message.

[MelissaPastore]

This might be overkill but I wasn't sure if we wanted to always apply the grid-row to the wrapper for the input or only do this when its parent is a form control element?