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/message_processor.rs

@@ -51,6 +51,7 @@ use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionSource;
 use futures::FutureExt;
 use tokio::sync::broadcast;
+use tokio::sync::watch;
 use tokio::time::Duration;
 use tokio::time::timeout;
 use toml::Value as TomlValue;
@@ -438,6 +439,11 @@ impl MessageProcessor {
             .await;
     }
 
+    pub(crate) fn subscribe_running_assistant_turn_count(&self) -> watch::Receiver<usize> {
+        self.codex_message_processor
+            .subscribe_running_assistant_turn_count()
+    }
+
     /// Handle a standalone JSON-RPC response originating from the peer.
     pub(crate) async fn process_response(&mut self, response: JSONRPCResponse) {
         tracing::info!("<- response: {:?}", response);