chore: update Vitest to 4.1.9 (#2587)

## Summary
- update all workspace `vitest` declarations to `4.1.9`
- update `@vitest/browser-playwright` and `@vitest/browser-webdriverio`
declarations to `4.1.9`
- refresh `pnpm-lock.yaml` so no `vitest`/`@vitest/browser` versions
below `4.1.9` remain

## Validation
- `sfw pnpm install`
- `pnpm format:check`
- `pnpm lint`
- `pnpm build`
- targeted audit check: no `vitest` or `@vitest/browser` advisories
remain
- `pnpm --filter create-langgraph test`
- `pnpm --filter @langchain/react test`
- `pnpm --filter @langchain/svelte test`
- `pnpm --filter @langchain/vue test`
- `pnpm --filter @langchain/angular test`
- `pnpm --filter @langchain/langgraph test`

## Notes
- `pnpm --filter @langchain/langgraph test:browser` did not run locally
because Playwright Chromium for the updated provider was not installed
on this machine; Vitest exited before running tests with Playwright's
`pnpm exec playwright install` message.
- The SDK Vitest suites passed but printed Vitest's post-success
close-timeout warning.

Co-authored-by: John Kennedy <jkennedyvz@users.noreply.github.com>