fix: normalize browser runner Playground calls

[chubes4]

Summary

• Normalize browser runner PHP execution helpers across Playground client request, file write, mkdir, and direct run API shapes.
• Prefer the worker endpoint { request } envelope for request-handler clients while retaining direct-client request compatibility.
• Add structured runtime error phase/code details and smoke coverage for worker endpoint and object-shaped file writes.

Fixes #567.

Tests

• node --check packages/wordpress-plugin/assets/browser-runtime.js
• php tests/smoke-wordpress-plugin.php
• npm run browser-runtime-operation-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Inspected the browser runtime contract, drafted the normalization changes and smoke coverage, and ran verification commands for Chris to review.