feat: support invalid selectable state

[Saithej2k]

Summary

• Add an isInvalid prop to EuiSelectable.
• Apply aria-invalid to both the searchable input and the options listbox.
• Keep listbox aria-invalid in sync when the prop changes.

API Changes

component / parent	prop / child	change	description
EuiSelectable	isInvalid	Added	Marks the search input and listbox as invalid for assistive technologies.

Impact Assessment

Impact level: Low. This adds an optional accessibility prop without changing default behavior.

Testing

• git diff --check
• yarn --cwd packages/eui eslint src/components/selectable/selectable.tsx src/components/selectable/selectable_list/selectable_list.tsx src/components/selectable/selectable.test.tsx src/components/selectable/selectable_list/selectable_list.test.tsx
• yarn --cwd packages/eui test-unit selectable.test.tsx selectable_list.test.tsx --runInBand
• git push pre-push checks

Closes #8991

[github-actions]

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