codex-rs/app-server: graceful websocket restart on Ctrl-C (#12517)

## Summary
- add graceful websocket app-server restart on Ctrl-C by draining until
no assistant turns are running
- stop the websocket acceptor and disconnect existing connections once
the drain condition is met
- add a websocket integration test that verifies Ctrl-C waits for an
in-flight turn before exit

## Verification
- `cargo check -p codex-app-server --quiet`
- `cargo test -p codex-app-server --test all
suite::v2::connection_handling_websocket`
- I (maxj) tested remote and local Codex.app

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -269,6 +269,7 @@ use std::time::SystemTime;
 use tokio::sync::Mutex;
 use tokio::sync::broadcast;
 use tokio::sync::oneshot;
+use tokio::sync::watch;
 use toml::Value as TomlValue;
 use tracing::error;
 use tracing::info;
@@ -2796,6 +2797,10 @@ impl CodexMessageProcessor {
             .await;
     }
 
+    pub(crate) fn subscribe_running_assistant_turn_count(&self) -> watch::Receiver<usize> {
+        self.thread_watch_manager.subscribe_running_turn_count()
+    }
+
     /// Best-effort: ensure initialized connections are subscribed to this thread.
     pub(crate) async fn try_attach_thread_listener(
         &mut self,