Bridge workspace-proxy WebSocket upgrades inside the Bun.serve listener so
the Hono path's WorkspaceRouterMiddleware → ServerProxy.websocket flow has
a native equivalent. The HttpApi handler still owns HTTP; the listener now
also resolves the workspace target inline (?workspace=… or session lookup),
upgrades the client connection, and bridges it to a remote WebSocket with
queueing, subprotocol forwarding, and close-code propagation.
This unblocks flipping Server.listen() over to the new listener but does
not flip it — the Hono path remains canonical. Bun-only; node:http + ws
adapter is a follow-up (TODO inline).
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