🤖 fix: #3617651 Edit rich text in the text format's own CKEditor 5

Problem

The translation editor bundled its own seven-plugin CKEditor. Markup the text format allows beyond those plugins (media embeds, headings, tables, data-entity-* link attributes, lang) was dropped and saved as the translation.

Fix

Rich text is now edited in the text format's own CKEditor 5, the same core editor Canvas uses.

• New GET /canvas-translate/api/editor-assets?formats=… returns the editor settings and JS/CSS that core would attach for those formats. ajax_page_state[libraries] keeps them from loading twice.
• The extension loads these assets into its iframe and attaches the editor through core's Drupal.editors.ckeditor5.
• Formats without a CKEditor 5 editor, or an editor that fails to load, fall back to editing as HTML.
• Bundled CKEditor packages removed: JS bundle 1.28 MB → 595 KB.

Tests

• Kernel: EditorAssetsControllerTest.
• Playwright: a heading, managed link, lang and <drupal-media> survive copy source → edit → publish. A second format loads its own editor without reloading core.

Out of scope

Translators can still edit formats they lack "use text format" for, as on 1.0.x (follow-up).

Closes #3617651.