app-server: thread resume subscriptions (#11474)

This stack layer makes app-server thread event delivery connection-aware
so resumed/attached threads only emit notifications and approval prompts
to subscribed connections.

- Added per-thread subscription tracking in `ThreadState`
(`subscribed_connections`) and mapped subscription ids to `(thread_id,
connection_id)`.
- Updated listener lifecycle so removing a subscription or closing a
connection only removes that connection from the thread’s subscriber
set; listener shutdown now happens when the last subscriber is gone.
- Added `connection_closed(connection_id)` plumbing (`lib.rs` ->
`message_processor.rs` -> `codex_message_processor.rs`) so disconnect
cleanup happens immediately.
- Scoped bespoke event handling outputs through `TargetedOutgoing` to
send requests/notifications only to subscribed connections.
- Kept existing threadresume behavior while aligning with the latest
split-loop transport structure.

codex-rs/app-server/src/message_processor.rs

@@ -396,9 +396,19 @@ impl MessageProcessor {
         }
     }
 
-    pub(crate) async fn try_attach_thread_listener(&mut self, thread_id: ThreadId) {
+    pub(crate) async fn try_attach_thread_listener(
+        &mut self,
+        thread_id: ThreadId,
+        connection_ids: Vec<ConnectionId>,
+    ) {
         self.codex_message_processor
-            .try_attach_thread_listener(thread_id)
+            .try_attach_thread_listener(thread_id, connection_ids)
+            .await;
+    }
+
+    pub(crate) async fn connection_closed(&mut self, connection_id: ConnectionId) {
+        self.codex_message_processor
+            .connection_closed(connection_id)
             .await;
     }