Style variations are saved without $schema #886

Description

Style variations created with "Save as style variation" don't get a $schema property. theme.json does: CBT_Theme_JSON_Resolver::export_theme_data() adds $schema and puts it first. CBT_Theme_JSON::add_theme_json_variation_to_local() writes $theme_json->get_data() plus the title, without a schema.

Without $schema, editors can't validate the file or autocomplete keys, and theme validation tools warn about it. The default themes declare it in every variation (for example, Twenty Twenty-Five's styles/01-evening.json).

Steps to reproduce

1. Activate Twenty Twenty-Five with Create Block Theme.
2. Create a style variation with Create Block Theme ("Save as style variation"), or call CBT_Theme_JSON::add_theme_json_variation_to_local( 'variation', array( 'name' => 'Schema Test', 'slug' => 'cbt-schema-test' ) ).
3. Open styles/cbt-schema-test.json.

Expected

The file starts with the same $schema that Create Block Theme writes into theme.json, followed by version.

Actual

{
	"version": 3,
	"title": "Schema Test"
}

Environment

• Create Block Theme 2.10.2
• WordPress 7.1.3, Gutenberg 24.2.0

Drafted with AI assistance, reproduced and verified by me.