Allow third-party color schemes to apply to Site Editor screens

PR #78397 made the Site Editor's sidebar and page shell follow the user's WordPress admin color scheme instead of always rendering a fixed dark background. This is a great improvement, but it currently only works for the built-in admin color schemes. Custom color schemes registered by plugins via `wp_admin_css_color()` are not picked up, since the colors are now sourced through `getAdminThemeColors`rather than from PHP's full list of registered admin color schemes. This breaks the experience for plugins that let users define and apply custom admin color schemes across wp-admin (like mine, [Admin Color Schemes](https://github.com/ryelle/admin-color-schemes)). Before #78397, it didn't matter, as the color schemes did not apply & were not visible; now that the admin bar is visible in the Site Editor and the core schemes apply, users of custom schemes get a visibly mismatched/inconsistent experience. ## Current behavior - `getAdminThemeColors` in `packages/admin-ui/src/admin-theme-colors` maps a fixed, known set of admin color scheme slugs to `primary`/`bg` values used by `ThemeProvider`. - Any scheme slug outside that known set (i.e. any custom scheme registered via `wp_admin_css_color()`) isn't represented, so the Site Editor falls back to a default rather than the plugin's colors. <img width="2048" height="1200" alt="Image" src="https://github.com/user-attachments/assets/baeb0426-f9bd-463a-8687-29fe3bddb599" /> ## Proposed solution Add a JS filter hook that lets plugins supply their custom scheme's colors to the Site Editor's theme color resolution. This could run in `getAdminThemeColors` to extend the list of `ADMIN_THEME_COLORS`. ## Additional context I have a PR in progress implementing this filter idea, and a branch to my admin color schemes plugin to demo using it — I'll post those shortly, but I'm opening this issue for any feedback on the filter idea. ## Environment - WordPress version: 7.1 beta or 7.0 + GB - Gutenberg version: 23.6