[EuiDelayRender] Migrate from class to function component

[Neilblaze]

Summary

Closes #9501

Converts EuiDelayRender from a class to a function component. The shouldComponentUpdate render gating becomes conditional rendering in the function body, and timer management moves to useEffect, as suggested in the issue's migration notes.

Behavior is preserved: children stay hidden on mount and are hidden again whenever they update, with repeated updates restarting the timer, so content only appears once updates have stopped for the configured delay. Two details worth calling out for review:

• Hiding updated children happens during the render phase (the documented storing information from previous renders pattern) rather than in the effect. Waiting for an effect would briefly commit updated children before hiding them again, flashing the content and triggering any aria-live announcements it may contain. That matters because the main internal consumer is the screen reader caption in EuiBasicTable. A dedicated test guards this and fails if the hide is moved into the effect.
• One intentional refinement: re-renders with referentially unchanged children no longer restart the delay, matching the React.memo semantics the issue suggests. The old class re-hid on every parent re-render because its shouldComponentUpdate never inspected props. All in-repo consumers pass fresh JSX children on every render, so nothing changes for them.

The component had no unit tests, so this adds a suite locking in the timing behavior. It was written against the class implementation first, and everything passes on the old class too except the unchanged children case described above.

delay becomes optional in the props type now that static defaultProps is gone. The runtime default of 500ms is unchanged.

API Changes

component / parent	prop / child	change	description
EuiDelayRender	delay	Type	Now optional in EuiDelayRenderProps (was required and filled by defaultProps). Runtime default of 500ms is unchanged

Impact Assessment

Internal refactor with no markup, class name, or styling changes. The only behavior difference is the unchanged children case above, which no known consumer relies on.

Impact level: 🟢 Low

Like the other migrations in this effort (#9860, #9879, #9831) this skips a changelog entry, so it needs the skip-changelog label from a maintainer.

QA instructions for reviewer

• Open the EuiDelayRender playground in Storybook.
  • The spinner appears only after the configured delay.
  • Changing args re-runs the delay before the spinner reappears.
  • The Cypress a11y spec (delay_render.a11y.tsx) passes.

[github-actions]

👋 Since this is a community submitted pull request, a Buildkite build has not been started automatically. Would an Elastic organization member please verify the contents of this pull request and kick off a build manually?

[Neilblaze]

CC'ing: @weronikaolejniczak for further visibility.

[Neilblaze]

cla/check

[weronikaolejniczak]

buildkite test this

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: 1b93129

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

• Buildkite Build
• Commit: 1b93129
• Documentation website
• Storybook
• ✅ Visual regression tests passed

[weronikaolejniczak]

LGTM 🟢 Thank you for contributing, @Neilblaze 🙏🏻