#3591340 Refactor AstroIsland

Dave Longrequested to merge
issue/canvas-3591340:3591340-refactor-astroisland-for into 1.x

Refactors the AstroIsland element to make it more readable and extensible.

  • New element properties: #inner_html to replace the HTML fragment inside the island wrapper, and #attributes to override the wrapper attributes.
  • Some helpers extracted for readability and final removed for the possibility of subclassing.
  • CANVAS_INTERNAL_PROP_KEYS constant added instead of repeating an array.
  • Test coverage for something untested that I broke during development.

AI-Generated: Partially (used Claude Code to help review some changes)

Testing instructions

  • Check that code components work as they did before

Closes #3591340

Merge request reports