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/src/server/process_handler.rs

@@ -3,7 +3,6 @@ use codex_app_server_protocol::JSONRPCErrorError;
 use crate::local_process::LocalProcess;
 use crate::protocol::ExecParams;
 use crate::protocol::ExecResponse;
-use crate::protocol::InitializeResponse;
 use crate::protocol::ReadParams;
 use crate::protocol::ReadResponse;
 use crate::protocol::TerminateParams;
@@ -28,19 +27,8 @@ impl ProcessHandler {
         self.process.shutdown().await;
     }
 
-    pub(crate) fn initialize(&self) -> Result<InitializeResponse, JSONRPCErrorError> {
-        self.process.initialize()
-    }
-
-    pub(crate) fn initialized(&self) -> Result<(), String> {
-        self.process.initialized()
-    }
-
-    pub(crate) fn require_initialized_for(
-        &self,
-        method_family: &str,
-    ) -> Result<(), JSONRPCErrorError> {
-        self.process.require_initialized_for(method_family)
+    pub(crate) fn set_notification_sender(&self, notifications: Option<RpcNotificationSender>) {
+        self.process.set_notification_sender(notifications);
     }
 
     pub(crate) async fn exec(&self, params: ExecParams) -> Result<ExecResponse, JSONRPCErrorError> {