feat: move exec-server ownership (#16344)

This introduces session-scoped ownership for exec-server so ws
disconnects no longer immediately kill running remote exec processes,
and it prepares the protocol for reconnect-based resume.
- add session_id / resume_session_id to the exec-server initialize
handshake
  - move process ownership under a shared session registry
- detach sessions on websocket disconnect and expire them after a TTL
instead of killing processes immediately (we will resume based on this)
- allow a new connection to resume an existing session and take over
notifications/ownership
- I use UUID to make them not predictable as we don't have auth for now
- make detached-session expiry authoritative at resume time so teardown
wins at the TTL boundary
- reject long-poll process/read calls that get resumed out from under an
older attachment

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

codex-rs/exec-server/src/rpc.rs

@@ -179,7 +179,8 @@ pub(crate) struct RpcClient {
 
 impl RpcClient {
     pub(crate) fn new(connection: JsonRpcConnection) -> (Self, mpsc::Receiver<RpcClientEvent>) {
-        let (write_tx, mut incoming_rx, transport_tasks) = connection.into_parts();
+        let (write_tx, mut incoming_rx, _disconnected_rx, transport_tasks) =
+            connection.into_parts();
         let pending = Arc::new(Mutex::new(HashMap::<RequestId, PendingRequest>::new()));
         let (event_tx, event_rx) = mpsc::channel(128);