Keep remote runtime mu-plugin URLs stable

[chubes4]

Summary

• Keep remote packaged browser runtime mu-plugin dependencies as stable package URLs instead of converting them into base64 data URLs.
• Preserve the visible Playground mu-plugin installer path while avoiding oversized response payloads for release ZIP inputs.
• Update smoke coverage to assert URL-based delivery for remote packaged mu-plugins.

Testing

• php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php
• npm run wordpress-plugin-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Investigated the oversized browser runtime payload path, drafted the focused WP Codebox change, and ran local verification; Chris remains responsible for review and merge.