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

---------

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

codex-rs/exec-server/tests/common/exec_server.rs

@@ -64,6 +64,17 @@ impl ExecServerHarness {
         &self.websocket_url
     }
 
+    pub(crate) async fn disconnect_websocket(&mut self) -> anyhow::Result<()> {
+        self.websocket.close(None).await?;
+        Ok(())
+    }
+
+    pub(crate) async fn reconnect_websocket(&mut self) -> anyhow::Result<()> {
+        let (websocket, _) = connect_websocket_when_ready(&self.websocket_url).await?;
+        self.websocket = websocket;
+        Ok(())
+    }
+
     pub(crate) async fn send_request(
         &mut self,
         method: &str,