fix(cypress): Chainlit shutdown and entrypoint checks

[coterp]

Summary

This PR improves the reliability of Cypress E2E runs when starting and stopping Chainlit.

Changes

• Track the spawned Chainlit process and shut it down reliably via process group
• Use port-based termination (fkill) only as a fallback when no tracked process exists
• Fix broken entrypoint file existence check (async fs/promises.access was not awaited)
• Improve signal handling and typing in cypress.config.ts
• Modernize Node core imports (node: + type imports)

Motivation

Cypress E2E runs were failing consistently due to unreliable Chainlit shutdown and restart behavior in containerized dev environments.

Tests frequently failed with port-in-use errors because previous Chainlit instances were not reliably terminated between runs or between specs. The existing port-based termination (fkill(:port)) proved unreliable in containers and was insufficient as a primary means of terminating Chainlit.

This PR introduces explicit process tracking and shutdown of the spawned Chainlit process, using port-based termination only as a fallback to clean up stray instances from previous runs.

Testing

• uv run pytest --cov=chainlit
• pnpm test (Cypress, headless via Xvfb)
• pnpm test:ui
• pnpm lint

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Summary by cubic

Improves reliability of Cypress E2E runs by cleanly starting and stopping Chainlit to avoid port-in-use errors in containers. Tracks and kills the spawned process group, using port-based termination only as a fallback.

• Bug Fixes
  • Fix entrypoint check by awaiting fs.promises.access.
  • Add stopChainlit and typed signal handling to kill the process group cleanly; fall back to fkill when no tracked process exists.
  • Streamline Cypress setup and restartChainlit flow; modernize Node imports and types.
  • On Windows, kill the entire Chainlit process tree with taskkill for reliable port release.

^{Written for commit d8e9a68. Summary will update automatically on new commits.}

[cubic-dev-ai]

No issues found across 2 files

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[asvishnyakov]

@coterp Thank you for contribution!

@hayescode @sandangel I would like to review this PR myself, as I tried to implement such functionality earlier and had an issues with that

[coterp]

@hayescode Sounds good, thanks for your review.
Let me know if you have any questions or want me to adjust anything.

[asvishnyakov]

@coterp Unfortunately I have same issue with it as when I tried implement such feature myself: it doesn't work in Cypress interactive mode, at least on Linux
Try to:

• Open Cypress in interactive mode (for example, via pnpm test:interactive)
• Run custom_theme tests (you'll see green screen, then red)
• Run data_layer tests (you'll see green screen again and tests will fail because login isn't enabled in previous test server, while it is in data_layer test server)

[asvishnyakov]

But I think we can merge some implements from here, like broken entrypoint file existence check, if you'll remove or fix new server restart code