chore(test): set up regression test suite to support React 17 upgrade (WIP)

[bosun-ai]

Summary

• Upgrade path initiated from React 16.8.6 toward React 17.0.2. This PR is a work in progress and focuses on setting up a regression test suite to support the upgrade.

Scope of changes

• Added a basic regression testing setup using Jest, React Testing Library, and MSW to validate key UI behaviors during the React upgrade.
  • .github/workflows/regression-tests.yml: Adds a GitHub Actions workflow to run regression tests on pushes/PRs to the alpha branch.
  • package.json: Adds test:regression script and testing dependencies.
  • regression-tests/jest.config.js and setupTests.js: Configures Jest environment and MSW server lifecycle.
  • regression-tests/button.test.js: Introduces an initial component regression test for Button behavior.
  • regression-tests/testServer.js and testHandlers.js: Provides API mocking via MSW for deterministic tests.

Reasoning

• Establishing a regression suite early helps catch breaking changes introduced by the React 17 upgrade, particularly around event handling, synthetic events, and DOM attribute differences.

Impact/Risks

• Minimal runtime impact; changes are isolated to testing and CI setup.
• CI will begin running regression tests on the alpha branch, which may surface pre-existing failures as we upgrade.

Regression test suite setup

• A Jest-based suite is configured under regression-tests/ with React Testing Library and MSW.
• CI: .github/workflows/regression-tests.yml runs npm run test:regression on pushes/PRs to alpha.

Testing instructions

• Run the full regression suite:
  • npm run test:regression
• Run a single component regression test:
  • npm run test:regression -- regression-tests/button.test.js

Follow-ups / Open questions

• Expand coverage to additional components affected by React 17 changes (e.g., event delegation, onClick semantics, label/for, and attribute normalization).
• Validate any custom portals or legacy lifecycle usage against React 17 behavior.
• Confirm Node and Jest versions are compatible with existing Babel/Jest config; adjust if needed during the upgrade.

Notes

• This is a work-in-progress PR to support the React 17 upgrade effort. Do not merge.

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