Expose responsive media as content-only editable

[chubes4]

Summary

• declare the generated responsive-media content attribute with Gutenberg role: content
• keep PHP block.json metadata and editor-side registration schemas identical
• execute the generated editor registration in Node to prevent schema drift

Closes #1081.

Verification

• cd php-transformer && composer test
• 280 PHP transformer parity fixtures pass
• package install proof passes
• focused generated PHP/JS attribute-schema parity passes

Blocks Engine does not currently include a browser-level Gutenberg harness, so the behavioral proof is the role contract consumed by WordPress contentOnly, not an editor interaction test.

AI assistance

• AI assistance: Yes
• Model: OpenAI gpt-5.6-sol
• Tool: OpenCode
• Used for: Tracing generated companion metadata, implementing matching PHP/JavaScript schemas, and adding executable parity coverage. Chris Huber reviewed the diff and remains responsible for every line.