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

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

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

@@ -5,6 +5,8 @@ mod collaboration_mode_list;
 mod compaction;
 mod config_rpc;
 mod connection_handling_websocket;
+#[cfg(unix)]
+mod connection_handling_websocket_unix;
 mod dynamic_tools;
 mod experimental_api;
 mod experimental_feature_list;