[EuiWindowEvent] Refactor class component to functional component

[barlima]

Summary

• What: Refactored EuiWindowEvent from a class component to a function component.
• Why: Fixes [EuiWindowEvent] Migrate from class to function component #9457 — part of the ongoing effort to migrate class components to functional components.
• How: Added EuiWindowEvent as a functional equivalent using useEffect with [event, handler] dependencies to match the class component's update logic (only re-registers the listener when event or handler change).

API Changes

No API changes

Screenshots

No visual changes

Impact Assessment

• [ ] 🔴 Breaking changes
• [ ] 💅 Visual changes
• 🧪 Test impact — Existing snapshots may need updating if the rendered output differs slightly.
• [ ] 🔧 Hard to integrate

Impact level: 🟢 None

Release Readiness

• Documentation:
• Figma:
• Migration guide:
• Adoption plan

QA instructions for reviewer

• Open the EuiFlyout stories in Storybook.
• Open a flyout and confirm it closes when pressing Escape.
• Open a flyout, click a focusable element inside it, and confirm Escape still closes the flyout (event listener is not lost on re-render).

Checklist before marking Ready for Review

• Filled out all sections above
• QA: Tested light/dark modes, high contrast, mobile, Chrome/Safari/Edge/Firefox, keyboard-only, screen reader
• QA: Tested in CodeSandbox and Kibana
• QA: Tested docs changes
• Tests: Added/updated Jest, Cypress, and VRT
• Changelog: Added changelog entry
• Breaking changes: Added breaking change label (if applicable)

Reviewer checklist

• Approved Impact Assessment — Acceptable to merge given the consumer impact.
• Approved Release Readiness — Docs, Figma, and migration info are sufficient to ship.

[github-actions]

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