Style variations: add $schema when saving variations - #890

What?

Make style variations declare $schema, matching theme.json and core default-theme variations, and unify the two code paths that write theme.json-shaped data.

Why?

Style variations created with "Create Theme Variation" were built from WP_Theme_JSON::get_data() plus a title, so they never declared $schema, while theme.json did. Without $schema, editors cannot validate or autocomplete the file and theme validators warn about it, and it is inconsistent with every core default theme, which declares $schema in each variation (e.g. Twenty Twenty-Five styles/01-evening.json).

The root cause is that two independent code paths write theme.json-shaped data and only one of them added the schema. Unifying them removes the drift instead of duplicating the logic.

Fixes #886.

How?

• Extract CBT_Theme_JSON_Resolver::get_schema() to resolve the schema URL: the theme's declared $schema, then the parent theme's for child themes, then the version-based URL.
• Extract CBT_Theme_JSON_Resolver::add_schema_to_data() to order $schema, version, and title first.
• Delegate export_theme_data() to the shared helpers so the export and variation paths cannot drift.
• Route CBT_Theme_JSON::add_theme_json_variation_to_local() through add_schema_to_data() before writing.
• Fix a latent key bug: the declared schema was read with array_key_exists( 'schema', ... ) instead of '$schema', so the theme's declared schema was effectively never used.
• Add tests/test-theme-json.php covering the ordering and asserting the variation's $schema matches theme.json.

Testing Instructions

1. Install dependencies with npm ci and composer install.
2. Start the test environment with npx wp-env start.
3. Run npm run test:unit:php.
4. Confirm Test_Create_Block_Theme_JSON passes, verifying that add_schema_to_data() orders $schema, version, and title first and that a created variation's $schema equals the one written to theme.json.
5. Run composer lint.
6. Manual check: in the Site Editor open Create Block Theme, create a theme variation, and confirm styles/<slug>.json starts with $schema then version.

Use of AI Tools

AI assistance: Yes
Tool: OpenAI Codex