Add managed previews for Links and Markdown library editors - #288213

Summary

This adds managed flyout editors with attached embeddable previews for Links and Markdown library items.

• adds openLazySystemFlyout while sharing lazy loading, overlay tracking, error handling, and focus restoration with the traditional flyout helper
• adds a reusable managed child flyout that renders an embeddable preview without breaking the EUI Flyout Manager context
• exposes Markdown saved objects in the Visualize Library and adds library editors for both Markdown and Links
• aligns editor actions around Cancel, Run preview, and Save/Apply, including a split-button action for saving by-value Links panels to the library
• preserves the editor when managed flyouts stack at narrow breakpoints, while allowing users to open the preview explicitly
• keeps Links previews content-sized and aligned to the top

Screenshots

Visualize Library

Links

Editor with attached preview	Preview closed

Narrow viewport, with the editor taking precedence:

Markdown

Editor with attached preview	Preview closed

Narrow viewport, with the editor taking precedence:

Dashboard editors

Links

By-value	Save-to-library action	By-reference

Markdown

By-value	By-reference

Testing

node scripts/check.js --scope=staged

• 165 Jest tests across 3 configurations
• 5 TypeScript projects
• ESLint and project configuration checks

Release note

Links and Markdown library items can now be edited from the Visualize Library with an attached live preview.

Checklist

• New UI text uses i18n and sentence case
• Unit tests were updated for the editor behavior
• No plugin configuration keys were changed
• No breaking HTTP API changes were introduced

Identify risks

The main risk is interaction between independently mounted system flyouts and the EUI Flyout Manager, especially during responsive layout changes. The implementation keeps React context intact, closes previews when the viewport narrows, guards async updates after unmount, and includes focused tests for the shared flyout and editor state behavior.