Add newWorkersWebSocketRpcSession for bidirectional server-side WebSockets + typescript fixes

[jmorrell]

• Add newWorkersWebSocketRpcSession — new export for bidirectional WebSocket RPC in Workers. Returns { response, remote } so server-side code can call back to the client with a typed proxy. newWorkersWebSocketRpcResponse is now a thin wrapper that discards the remote.

• Unify tsconfig — single tsconfig.json covers both library and test files so the editor/language server picks up Cloudflare Workers types. Add npm run typecheck script.

• Eliminate as any from production code — replace with Record<string, unknown> casts in type guards. Reduce test as any from 20 to 5 justified instances.

  Before/after: bidirectional worker fixture

  // Before (20 lines, manual plumbing, untyped)
  if (request.headers.get("Upgrade") !== "websocket") {
    return new Response("Expected WebSocket upgrade", { status: 400 });
  }
  const pair = new WebSocketPair();
  const [client, server] = Object.values(pair);
  server.accept();
  const transport = createWebSocketTransport(server);
  const biService = {
    async addWithClientMultiplier(a: number, b: number): Promise<number> {
      const multiplier: number = await (session.remote as any).getMultiplier();
      return (a + b) * multiplier;
    },
  };
  const session = new RpcSession(transport, biService, { role: "acceptor" });
  return new Response(null, { status: 101, webSocket: client });

  // After (8 lines, fully typed)
  const biService = {
    async addWithClientMultiplier(a: number, b: number): Promise<number> {
      const multiplier = await remote.getMultiplier();
      return (a + b) * multiplier;
    },
  };
  const { response, remote } = newWorkersWebSocketRpcSession<ClientService>(request, biService);
  return response;