TextInput accessibility fixes

[joshfarrant]

Summary

Fixes all outstanding accessibility issues for the TextInput component.

List of notable changes:

• Add autoComplete attribute to relevant TextInput components
• Update TextInput documentation to explicitly mention the autoComplete attribute
• Add aria-describedby to TextInput component
• Add tests for TextInput component
• Remove validation prop from TextInput stories
  • The prop doesn't really make sense in the TextInput stories here as it's intended to be used with FormControl.Validation, so it's been removed to avoid confusion.
• Add aria-labels to TextInput leadingVisual and trailingVisual icons in Storybook
• Modify TextInput documentation to not encourage placeholder use as label

What should reviewers focus on?

• Check that I've not made any mistakes on the autoComplete attributes
• Check that the aria-describedby attribute is correctly implemented

Steps to test:

Link to Storybook

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

• Closes https://github.com/github/primer/issues/3808
• Closes https://github.com/github/primer/issues/3776
• Closes https://github.com/github/primer/issues/3809
• Closes https://github.com/github/primer/issues/3792

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:

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: bae7b7c

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-e2e	Patch
@primer/brand-primitives	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.

Please review the differences by using the test artifacts to ensure that the changes were intentional.

Artifacts can be downloaded and reviewed locally.

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Example:

If the changes are expected, please run npm run test:visual:update-snapshots to replace the previous fixtures.

Review visual differences

[danielguillan]

Looks great! Thank you!