test(vitest): enable console enforcement and document env override

[joshblack]

Enables the shared Vitest console enforcement by default for every Vitest project, and documents the VITEST_FAIL_ON_CONSOLE=false local debugging override in contributor docs and agent instructions.

Changelog

New

• N/A

Changed

• Vitest now fails by default when tests emit unexpected console output.
• Added documentation for the fail-on-console environment variable helper in:
  • contributor-docs/testing.md
  • .github/copilot-instructions.md

Removed

• N/A

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; test infrastructure + documentation updates only.

Testing & Reviewing

Validated with build, tests, type-check, lint, CSS lint, and format checks before the docs follow-up.
Validated docs follow-up with npm run lint and npm run format:diff.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

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Latest commit: b6b6345

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[Copilot]

Pull request overview

This PR updates the Vitest project configurations across the monorepo to enable the shared @primer/vitest-config console enforcement by default, while still allowing local opt-out via VITEST_FAIL_ON_CONSOLE=false.

Changes:

• Flip the __VITEST_FAIL_ON_CONSOLE__ default from opt-in (=== 'true') to opt-out (!== 'false') across all Vitest project configs.
• Apply the same behavior consistently for both node and browser Vitest projects.

Show a summary per file

File	Description
packages/styled-react/vitest.config.ts	Enable fail-on-console by default for styled-react node tests.
packages/styled-react/vitest.config.browser.ts	Enable fail-on-console by default for styled-react browser tests.
packages/react/vitest.config.mts	Enable fail-on-console by default for react node tests.
packages/react/vitest.config.browser.mts	Enable fail-on-console by default for react browser tests.
packages/postcss-preset-primer/vitest.config.ts	Enable fail-on-console by default for postcss-preset-primer tests.
packages/doc-gen/vitest.config.mts	Enable fail-on-console by default for doc-gen tests.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 0

[joshblack]

@copilot can you add docs for this to agent instructions and contributor docs? Specifically the fail on console ENV helper

[Copilot]

@copilot can you add docs for this to agent instructions and contributor docs? Specifically the fail on console ENV helper

Done in b6b6345.

I added docs for the fail-on-console ENV helper in:

• .github/copilot-instructions.md (agent instructions)
• contributor-docs/testing.md (contributor testing docs)

Both now document that console enforcement is on by default and that VITEST_FAIL_ON_CONSOLE=false can be used for local debugging.