Fix broken language service and double-execution in playground

[minestarks]

The playground the language service was non-functional, and the compiler was silently double-executing every request.

What was happening:

With #3153 :

• The BrowserWorkerHost blob bootstrap registered message handlers via self.addEventListener("message", handler)
• Consumers (like the playground) also assign self.onmessage = messageHandler
• Both fire on every message, so initService ran twice (crashing on the second initLogging call) and every subsequent request produced duplicate responses that confused the proxy

Fix:

• Switch the blob's onMessage implementation to self.onmessage = handler — now when a consumer reassigns self.onmessage with the same handler, it's just a harmless overwrite instead of a second registration
• No changes needed in downstream consumers