feat(combobox): add imperative ref handle for programmatic close

[jtomaszewski]

Summary

Add ComboboxHandle type with close() method, allowing programmatic control of the Combobox via useImperativeHandle.

Fixes #3649

Usage

import { Combobox, ComboboxHandle } from '@headlessui/react'
import { useRef } from 'react'

function Example() {
  const comboboxRef = useRef<ComboboxHandle>(null)

  return (
    <>
      <button onClick={() => comboboxRef.current?.close()}>
        Close Combobox
      </button>
      <Combobox ref={comboboxRef}>
        <ComboboxInput />
        <ComboboxOptions>
          <ComboboxOption value="a">Option A</ComboboxOption>
        </ComboboxOptions>
      </Combobox>
    </>
  )
}

Test plan

• Added tests for closing via ref.current.close()
• Verified onClose callback is triggered
• All existing combobox tests pass

🤖 Generated with Claude Code

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[ignatremizov]

This is not backwards compatible (ref was HTMLElement type which you replace with ComboboxHandle) - if you need this just expose it as another prop