Fix TreeView React 19 test timer leakage

[Copilot]

Changelog

New

Changed

• Updated packages/react/src/TreeView/TreeView.test.tsx to reset Vitest timers to real timers before each test, preventing React 19 browser tests from inheriting fake timers from earlier suites.
• Fixes the failing test (react-19) GitHub Actions job by preventing the first TreeView browser test from hanging and causing cascading empty-DOM failures.

Removed

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Test-only fix for CI stability; no production behavior or API changes.

Testing & Reviewing

• Reviewed Actions logs for run 25460332110, job 74702059950, and confirmed the failure started in packages/react/src/TreeView/TreeView.test.tsx after fake timers leaked into the React 19 browser run.
• Verified the fix with targeted TreeView browser tests.
• Verified a React 19 targeted subset after node script/setup-react-19.mts.
• Ran:
  • npm test
  • npm run build
  • npm run type-check
  • npm run lint
  • npm run lint:css
  • 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)

[changeset-bot]

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Latest commit: e45519c

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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]

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