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/main.rs

@@ -17,7 +17,7 @@ const DISABLE_MANAGED_CONFIG_ENV_VAR: &str = "CODEX_APP_SERVER_DISABLE_MANAGED_C
 #[derive(Debug, Parser)]
 struct AppServerArgs {
     /// Transport endpoint URL. Supported values: `stdio://` (default),
-    /// `ws://IP:PORT`, `off`.
+    /// `unix://`, `unix://PATH`, `ws://IP:PORT`, `off`.
     #[arg(
         long = "listen",
         value_name = "URL",