feat(harness-cursor): implement Cursor harness adapter (#19373)

## Background

Cursor is the remaining coding-agent CLI from the original ACP harness
proposal that does not have a first-party AI SDK harness adapter.

## Summary

This adds a Cursor adapter on top of `harness-acp`, while preserving
Cursor-specific installation, authentication, and tool behavior.

- Add `harness-cursor`, installed through Cursor's official install
script and launched over ACP.
- Map Cursor built-in and MCP tools, broker `CURSOR_API_KEY`, and
surface account-configured provider routing clearly.
- Add unit and type coverage, adapter documentation, and the standard
`ai-functions` and Next.js end-to-end example matrices.
- Document unsupported Cursor ACP capabilities, including structured
output and built-in tool filtering.

## End-to-End Verification

Ran the added examples in
`examples/ai-functions/src/harness-agent/cursor` and the interactive
examples for Cursor in `examples/harness-e2e-next`.

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [ ] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Future Work

Add native instruction and structured-output mappings if or once Cursor
exposes corresponding ACP extensions.

## Related Issues

See #16956