Updated the accessible label of the SubNav's open/close toggle button to include the current page

[joshfarrant]

Summary

Following on from this discussion, I spotted a Sev-2 a11y issue which references the same underlying issue.

• Fixes the above issue in the suggested way by removing the aria-label and instead adding visually-hidden text which describes the control and states the current page.
• Removes test-ids from the SubNav's tests in favour of more accessibility-focussed queries
• Replaces fireEvent usage in those tests with userEvent as we were previously using both in tests unnecessarily.

What should reviewers focus on?

• Check you're happy with the approach
• Check you're happy with the test changes

Steps to test:

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

• Closes https://github.com/github/primer/issues/3799

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
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• 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

[changeset-bot]

🦋 Changeset detected

Latest commit: 68ae6ba

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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Copilot reviewed 2 out of 2 changed files in this pull request and generated no suggestions.

Comments skipped due to low confidence (1)

packages/react/src/SubNav/SubNav.test.tsx:114

• [nitpick] The test case name should be updated to reflect the new functionality. Consider renaming it to 'it('includes the aria-current text in the aria-label of the button', () => {'

it('shows the aria-current text next to the button by default', () => {

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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.

Download links are available at the bottom of the workflow summary screen.

Example:

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

Review visual differences

[rezrah]

Thanks @joshfarrant ✨