Stabilize websocket test server binding (#14002)

## Summary
- stop reserving a localhost port in the websocket tests before spawning
the server
- let the app-server bind `127.0.0.1:0` itself and read back the actual
bound websocket address from stderr
- update the websocket test helpers and callers to use the discovered
address

## Why this fixes the flake
The previous harness reserved a port in the test process, dropped it,
and then asked the server process to bind that same address. On busy
runners there is a race between releasing the reservation and the child
process rebinding it, which can produce sporadic startup failures.
Binding to port `0` inside the server removes that race entirely, and
waiting for the server to report the real bound address makes the tests
connect only after the listener is actually ready.

codex-rs/app-server/tests/suite/v2/command_exec.rs

@@ -29,7 +29,6 @@ use super::connection_handling_websocket::assert_no_message;
 use super::connection_handling_websocket::connect_websocket;
 use super::connection_handling_websocket::create_config_toml;
 use super::connection_handling_websocket::read_jsonrpc_message;
-use super::connection_handling_websocket::reserve_local_addr;
 use super::connection_handling_websocket::send_initialize_request;
 use super::connection_handling_websocket::send_request;
 use super::connection_handling_websocket::spawn_websocket_server;
@@ -712,8 +711,7 @@ async fn command_exec_process_ids_are_connection_scoped_and_disconnect_terminate
     let codex_home = TempDir::new()?;
     create_config_toml(codex_home.path(), &server.uri(), "never")?;
 
-    let bind_addr = reserve_local_addr()?;
-    let mut process = spawn_websocket_server(codex_home.path(), bind_addr).await?;
+    let (mut process, bind_addr) = spawn_websocket_server(codex_home.path()).await?;
 
     let mut ws1 = connect_websocket(bind_addr).await?;
     let mut ws2 = connect_websocket(bind_addr).await?;