fix: featured snippets retrieval against the snippets-only cloud response

[imantsk]

Summary

Featured snippets returned empty after the cloud-app dropped the redundant data alias from search/featured responses (now snippets-only).

Cloud_API::get_featured_snippets() had its own inline parse requiring isset($json['data']), separate from the shared unpack_request_json() helper, so it returned an empty Cloud_Snippets once data was gone.

Changes

• get_featured_snippets() now parses via unpack_request_json() (the full envelope), matching the search path.
• Cloud_Snippets reads snippets from the snippets key only; the legacy data fallback is removed.
• Taxonomy parsers (get_cloud_types/get_cloud_categories) are unchanged — those endpoints still return data.
• Test fixtures for featured/search now mirror the real snippets-only response.

Verification

• phpunit --group cloud: 30/30 pass.
• Live against staging (snippets-only): featured returns 10/10 snippets (was 0 before the fix).