Twenty Twenty-Five: Ensure Editor styles are enqueued.

This changeset replaces get_parent_theme_file_uri( 'assets/css/editor-style.css' ) with add_editor_style( 'assets/css/editor-style.css' ); to ensure a relative path to the stylesheet file is used, and not an URL. This prevents the editor stylesheet from not being enqueued on various edge cases.

Merges [60424] to the 6.8 branch.
Props wildworks, sabernhardt, dhruvang21, karmatosed, SirLouen, audrasjb, sandeepdahiya.
Fixes #63399.