Convert EuiMutationObserver from class to function component

[ragini-pandey]

Summary

Converts EuiMutationObserver from a class component (extending EuiObserver) to a standalone function component, as requested in #9459.

Key implementation details:

• Inlined the EuiObserver lifecycle logic (ref tracking via updateChildNode, observer connect/disconnect) directly into the function component using useRef and useCallback
• Both onMutation and observerOptions are stored in refs so the updateChildNode callback identity stays stable across renders — this prevents React from cycling the ref callback (disconnect → reconnect), which would cause missed mutations
• Cleanup on unmount is handled via a useEffect with an empty dependency array
• The makeMutationObserver helper and useMutationObserver hook are unchanged
• The EuiObserver base class is left intact since EuiResizeObserver still extends it
• No changes to public API — EuiMutationObserver, EuiMutationObserverProps, and useMutationObserver exports remain identical

Why are we making this change?

Closes #9459 — Migrating class components to function components for consistency and to leverage React hooks.

Impact to users

No impact. This is a pure internal refactor — the public API, behavior, and rendered output are identical.

QA

General checklist

• Browser QA
  • Checked in both light and dark modes (no visual changes)
  • Checked in both MacOS and Windows high contrast modes (no visual changes)
  • Checked in mobile (no visual changes)
  • Checked in Chrome, Safari, Edge, and Firefox (no visual changes)
  • Checked for accessibility (no visual changes)
• Docs site QA
  • Added documentation (no API changes)
  • Props have proper autodocs (no API changes)
  • Checked Code Sandbox (no API changes)
• Code quality checklist
  • Added or updated jest tests — existing tests pass with no modifications
  • Updated visual regression tests (no visual changes)
• Release checklist
  • A changelog entry exists and is marked appropriately
  • Breaking change (not applicable)

[github-actions]

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[Copilot]

Pull request overview

Refactors EuiMutationObserver from a class component (via EuiObserver) into a standalone function component implemented with React hooks, while aiming to preserve the existing API and behavior.

Changes:

• Replaced the EuiObserver inheritance model with hook-based ref tracking and observer lifecycle management inside EuiMutationObserver.
• Kept makeMutationObserver and useMutationObserver intact.
• Added an upcoming changelog entry documenting the refactor.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
packages/eui/src/components/observer/mutation_observer/mutation_observer.tsx	Converts EuiMutationObserver to a hook-based function component and inlines observer lifecycle logic.
packages/eui/changelogs/upcoming/9508.md	Adds a changelog entry for the component refactor.

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[ragini-pandey]

Closing as per #9511 (comment)