Closes #4772

Problem

handle() from hono/lambda-edge returns an async handler with Function.length === 3 because callback is a formal parameter. AWS Lambda NODEJS_24_X determines handler type by Function.length — when it's >= 3, the runtime treats it as a callback-style handler and throws Runtime.CallbackHandlerDeprecated.

Solution

Replace the three formal parameters (event, context, callback) with (event, ...args) and destructure context and callback internally. This reduces Function.length to 1 while preserving positional callback compatibility for older runtimes.

Before

return async (event: CloudFrontEdgeEvent, context?: CloudFrontContext, callback?: Callback) => {
  // Function.length === 3 → NODEJS_24_X treats as callback-style
}

After

return async (event: CloudFrontEdgeEvent, ...args: [CloudFrontContext?, Callback?]) => {
  const [context, callback] = args
  // Function.length === 1 → NODEJS_24_X treats as async-style
}

Tests Added

• Verify handler.length <= 2 (regression test for the root cause)
• Verify positional callback still works via handler(event, undefined, callback)

it('Should expose async handler arity compatible with NODEJS_24_X', () => {
  expect(handler.length).toBeLessThanOrEqual(2)
})

it('Should preserve positional callback compatibility', async () => {
  type Env = { Bindings: { callback: Callback } }
  const app = new Hono<Env>()
  const callback = vi.fn()

  app.get('/test-path', (c) => {
    c.env.callback?.(null, {
      status: '200',
      headers: {
        'x-test': [{ key: 'x-test', value: 'ok' }],
      },
    })
    return c.text('ok')
  })

  const handler = handle(app)
  await handler(cloudFrontEdgeEvent, undefined, callback)

  expect(callback).toHaveBeenCalledWith(null, {
    status: '200',
    headers: {
      'x-test': [{ key: 'x-test', value: 'ok' }],
    },
  })
})

Testing Note

• This fix directly addresses the handler.length issue, which is the documented cause of the Runtime.CallbackHandlerDeprecated error in the NODEJS_24_X runtime.
• Since this involves AWS infrastructure and I don't have an environment ready for immediate end-to-end testing, verification on a real deployment would be greatly appreciated.

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code