[KZN-4067]: expose ref on Collapsible

[AliceMenzie]

Summary

• Wraps Collapsible in forwardRef, forwarding the ref to the toggle IconButton via useImperativeHandle
• Exposes a ButtonRef interface ({ focus: () => void }) so consumers can programmatically focus the toggle button
• Re-exports ButtonRef type from the Collapsible index for convenience

Context

Requested in KZN-4067. The consumer (probations-reviews-ui) needed to restore focus to the Collapsible toggle button after async operations (save/delete in a sortable list) and was working around this with a fragile querySelector('[data-testid="collapsible-button-..."]').

Usage

import { useRef } from 'react'
import { Collapsible, type ButtonRef } from '@kaizen/components'

const collapsibleRef = useRef<ButtonRef>(null)

// After an async operation:
collapsibleRef.current?.focus()

<Collapsible ref={collapsibleRef} title="Settings">
  {/* content */}
</Collapsible>

Test plan

• All existing Collapsible tests pass
• New ref tests cover: ref defined, focus callable, controlled mode, uncontrolled mode, fallback id, useRef hook pattern
• Storybook story WithProgrammaticFocus demonstrates the async focus flow

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🦋 Changeset detected

Latest commit: 2982805

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@kaizen/components	Patch

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Last updated for commit 2982805: refactor(Collapsible): replace ButtonRef with CollapsibleRef