docs: fix contributor setup and test commands (#2717)

## Summary

- fix the npm package link and repository directory used in the
contributor setup
- replace the removed `test:single` instructions with the current pnpm
workspace and Vitest commands
- point integration-test setup to the existing
`libs/langgraph-core/.env.example`

## Testing

- `pnpm --filter @langchain/langgraph build`
- `pnpm --filter @langchain/langgraph test src/graph/message.test.ts` (4
tests passed)
- `pnpm --filter @langchain/langgraph exec vitest list --mode int
src/tests/chatbot.int.test.ts --filesOnly`
- `pnpm format:check`

Docs-only change. No public API changes and no changeset required.

Co-authored-by: To3akaRin <To3akaRin@users.noreply.github.com>
Co-authored-by: Christian Bromann <git@bromann.dev>