fix(deno): stabilize reconnect integration test (#567)

## Summary

Stabilizes a flaky Deno provider integration test that intermittently
failed in CI when reconnecting to an existing sandbox immediately after
closing the original connection. The reconnect path can briefly return a
transient `404 DEPLOYMENT_NOT_FOUND`, causing otherwise healthy runs to
fail. This change adds bounded retry/backoff during reconnect and
ensures the reconnected sandbox handle is explicitly closed.

## Changes

### `@langchain/deno`

- Updated reconnect integration test in
`libs/providers/deno/src/sandbox.int.test.ts` to retry
`DenoSandbox.fromId(sandboxId)` with `withRetry` (`5` attempts, `1s`
delay).
- Added a short comment documenting the transient reconnect window after
WebSocket close.
- Added explicit cleanup for the reconnected sandbox to avoid leaking
extra connections during the test run.