feat(langchain/agents): canJumpTo hook param

[Hunter Lovell (hntrl)]

before (yuck, gross):

createMiddleware({
  ...,
  beforeModelJumpTo: [...],
  beforeModel: () => {}
});

after (woo, yay):

createMiddleware({
  ...,
  beforeModel: {
    canJumpTo: [...],
    hook: () => {}
  }
});

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