app-server: add Unix socket transport (#18255)

## Summary
- add unix:// app-server transport backed by the shared codex-uds crate
- reuse the websocket connection loop for axum and tungstenite-backed
streams
- add codex app-server proxy to bridge stdio clients to the control
socket
- tolerate Windows UDS backends that report a missing rendezvous path as
connection refused before binding

## Tests
- cargo test -p codex-app-server
control_socket_acceptor_forwards_websocket_text_messages_and_pings
- cargo test -p codex-app-server
- just fmt
- just fix -p codex-app-server
- git -c core.fsmonitor=false diff --check

codex-rs/app-server/src/message_processor.rs

@@ -362,7 +362,7 @@ impl MessageProcessor {
         self: &Arc<Self>,
         connection_id: ConnectionId,
         request: JSONRPCRequest,
-        transport: AppServerTransport,
+        transport: &AppServerTransport,
         session: Arc<ConnectionSessionState>,
     ) {
         let request_method = request.method.as_str();