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/tracing_tests.rs

@@ -188,7 +188,7 @@ impl TracingHarness {
             .process_request(
                 TEST_CONNECTION_ID,
                 request,
-                AppServerTransport::Stdio,
+                &AppServerTransport::Stdio,
                 Arc::clone(&self.session),
             )
             .await;
@@ -211,7 +211,7 @@ impl TracingHarness {
             .process_request(
                 TEST_CONNECTION_ID,
                 request,
-                AppServerTransport::Stdio,
+                &AppServerTransport::Stdio,
                 Arc::clone(&self.session),
             )
             .await;