Addon A11y: Refactor environment variable handling for Vitest integration

[valentinpalkovic]

Closes #

What I did

Accessibility checks were always running, independently of whether the a11y checkbox was checked in the testing module or not. The applied fix applies environment variables correctly again to avoid the issue.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

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Greptile Summary

Based on the provided files and PR context, I'll create a concise summary of the key changes:

Refactored environment variable handling in Storybook's a11y and test addons to fix accessibility checks running unintentionally.

• Simplified environment checks in code/addons/a11y/src/utils.ts to directly use import.meta.env instead of redundant process.env fallbacks
• Added conditional a11y check enablement in code/addons/test/src/vitest-plugin/index.ts based on environment configuration
• Moved VITEST_STORYBOOK environment variable definition earlier in the code to avoid duplication
• Removed unused TestManager import and unnecessary process.env polyfill from define section

The changes ensure accessibility checks only run when explicitly enabled through user settings, fixing a bug where they would always execute regardless of configuration.

[greptile-apps]

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