Reapply "Add app-server transport layer with websocket support" (#11370)

Reapply "Add app-server transport layer with websocket support" with
additional fixes from https://github.com/openai/codex/pull/11313/changes
to avoid deadlocking.

This reverts commit 47356ff83c.

## Summary

To avoid deadlocking when queues are full, we maintain separate tokio
tasks dedicated to incoming vs outgoing event handling
- split the app-server main loop into two tasks in
`run_main_with_transport`
   - inbound handling (`transport_event_rx`)
   - outbound handling (`outgoing_rx` + `thread_created_rx`)
- separate incoming and outgoing websocket tasks

## Validation

Integration tests, testing thoroughly e2e in codex app w/ >10 concurrent
requests

<img width="1365" height="979" alt="Screenshot 2026-02-10 at 2 54 22 PM"
src="https://github.com/user-attachments/assets/47ca2c13-f322-4e5c-bedd-25859cbdc45f"
/>

---------

Co-authored-by: jif-oai <jif@openai.com>

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

@@ -1,4 +1,6 @@
-use codex_app_server::run_main;
+use clap::Parser;
+use codex_app_server::AppServerTransport;
+use codex_app_server::run_main_with_transport;
 use codex_arg0::arg0_dispatch_or_else;
 use codex_core::config_loader::LoaderOverrides;
 use codex_utils_cli::CliConfigOverrides;
@@ -8,19 +10,34 @@ use std::path::PathBuf;
 // managed config file without writing to /etc.
 const MANAGED_CONFIG_PATH_ENV_VAR: &str = "CODEX_APP_SERVER_MANAGED_CONFIG_PATH";
 
+#[derive(Debug, Parser)]
+struct AppServerArgs {
+    /// Transport endpoint URL. Supported values: `stdio://` (default),
+    /// `ws://IP:PORT`.
+    #[arg(
+        long = "listen",
+        value_name = "URL",
+        default_value = AppServerTransport::DEFAULT_LISTEN_URL
+    )]
+    listen: AppServerTransport,
+}
+
 fn main() -> anyhow::Result<()> {
     arg0_dispatch_or_else(|codex_linux_sandbox_exe| async move {
+        let args = AppServerArgs::parse();
         let managed_config_path = managed_config_path_from_debug_env();
         let loader_overrides = LoaderOverrides {
             managed_config_path,
             ..Default::default()
         };
+        let transport = args.listen;
 
-        run_main(
+        run_main_with_transport(
             codex_linux_sandbox_exe,
             CliConfigOverrides::default(),
             loader_overrides,
             false,
+            transport,
         )
         .await?;
         Ok(())