Playground: Fix errors not shown on page load

[MichaReiser]

Summary

This PR fixes a bug in the playground where errors for a restored snipped didn't show on page load.

The core issue was that editor or editor.getModels was None when the diagnostics are updated, in which case the useEffect skips the diagnostics change.

This PR fixes the issue by:

• Using onMount instead of beforeMount to only set the editor instance when the editor is fully loaded
• Setting the diagnostics as part of the handleMount callback because the mount event could be triggered after are updated

I used this as a chance to only register a single code action provider.

Fixes #11918

Test Plan

Screencast.from.2024-09-06.10-00-24.webm