Fix browser PHP request handler usage

[chubes4]

Summary

• Prefer the Playground PHP request handler when running browser-side PHP requests.
• Keep the legacy client request path as a fallback for older clients.
• Add browser runtime smoke coverage proving the request handler path does not call the deprecated client request method.

Testing

• npm run browser-runtime-operation-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the browser runtime fix, added smoke coverage, and ran targeted verification; Chris requested the change and merge.